fix: stop shrinking conversations behind the user's back — both automatic context guards ship off

The shipped default combined a sliding history window with no compaction, which
is the worse of the two available trades in both directions it is measured on.

`max_history_messages: 30` is a sliding tail window (`projection::window` —
`drain(..len - max)`). Past 30 messages it drops from the head on *every* turn,
so the prompt prefix changes on every single request and every provider that
caches one (Anthropic breakpoints, OpenAI automatic prefix caching) misses every
time. It also drops those messages with no summary standing in for them: silent
amnesia, not just a cold cache. Compaction rewrites the prefix once per
compaction and leaves a summary behind — yet it was the half that was commented
out, while the window's own doc-comment already said the two were exclusive.

Both are now `Option` and both ship unset, so nothing shrinks a conversation
unless a human types `/compact`.

Which surfaced the real bug: `/compact` did not work either. The compactor was
`Option<Arc<ContextCompactor>>` keyed on the config section existing, so
commenting out `compaction:` disabled the manual command too — `force_compact`
returned `Ok(false)` and the chat answered "compaction disabled". Manual
compaction is a command a user types; it cannot depend on an admin having filled
in a token threshold. The compactor is now built unconditionally and
`threshold_tokens: Option<u32>` arms only the automatic pass; `try_compact`
early-returns without it, `force_compact` deliberately never consults it.

The projection accordingly yields to the *automatic* pass rather than to the
compactor's existence (`LoopConfig.auto_compaction_enabled`), so a configured
message cap is not silently voided by `/compact` merely being available.
`CompactionConfig::Default` is hand-written for the same reason `RoleAttrs`'s is:
a derived one gives `keep_recent: 0`, which would compact away every recent
message on any box omitting the section — now the default.

Also fixes two documentation bugs in the same file: `event_triage` was documented
nested under `llm:`, where it parses fine and is then silently ignored (it is a
top-level field), and `datetime` was documented twice with conflicting examples.

A new test asserts the shipped default actually deserializes and that both guards
are off — a field the default omits must be genuinely optional, or a brand-new
install fails to boot.

Automatic compaction returns later, triggered off the resolved model's own
context window instead of a hand-tuned token count that cannot know which model
is answering.

CLAUDE.md

@@ -109,7 +109,7 @@ Two rules keep the boundary real, and both are enforced by the compiler:
 | `crates/skald-core/src/cron/` | Scheduled job runner |
 | `crates/skald-core/src/system_agents/` | The `SystemAgent` trait + `run_and_record` + the shared ephemeral-turn/run-context machinery, plus `registry()` (the one enumeration of the agents) and `memory_lint.rs` (the two lint agents). See the system-agents section |
 | `crates/skald-core/src/event_triage/` | `EventTriageManager`: one pass of the event-triage system agent for **one** user. No timer of its own — the instance-wide scheduler is `skald::wiring::spawn_system_agents` |
-| `crates/skald-core/src/compactor.rs` | Context compaction **policy** — when to compact and with which model; the mechanics are `agent_loop::compaction`. Model for the summary call: the instance-wide Settings pick (`compaction_model`, a `PropertyType::LlmModel` config property declared by `compactor::config_set`) wins; else AUTO by `compaction.strength` (config.yml); a missing configured model degrades to the same AUTO path |
+| `crates/skald-core/src/compactor.rs` | Context compaction **policy** — when to compact and with which model; the mechanics are `agent_loop::compaction`. The compactor is **always constructed** (manual `/compact` must work with no config); `compaction.threshold_tokens` is `Option` and arms only the *automatic* pass, and is **unset by default** — see the context-size defaults section. Model for the summary call: the instance-wide Settings pick (`compaction_model`, a `PropertyType::LlmModel` config property declared by `compactor::config_set`) wins; else AUTO by `compaction.strength` (config.yml); a missing configured model degrades to the same AUTO path |
 | `crates/skald-core/src/approval/` | Approval rules engine |
 | `crates/skald-core/src/clarification/` | `ClarificationManager`: background-session question/answer |
 | `crates/skald-core/src/elicitation/` | `ElicitationManager` + bridge: MCP server-initiated input (`elicitation/create`), surfaced in the Inbox; secrets never logged/persisted |
@@ -350,6 +350,20 @@ A crash loses RAM (the approval oneshot, the cancellation token), never truth: e
 
 `agent_loop::compaction` owns the mechanics: split point (never between an assistant turn and its tool results), transcript, prompt (`SUMMARY_PREFIX` / preamble / template live there now), the single no-tools model call, the saved summary row. `skald-core/src/compactor.rs` owns the **policy**: the token threshold, the ephemeral guard, which model summarises (`compaction_model` from Settings, else AUTO by `compaction.strength`), and publishing `CompactionEvent` on the chat bus. The DTL re-anchor is the `on_compacted` hook (`loop_adapters/hooks.rs::DtlReanchorHook`). The next turn needs nothing: the assembler reads the latest summary from the store.
 
+### Context size: both automatic guards are off by default
+
+Nothing shrinks a conversation unless a human asks. `llm.max_history_messages` and `llm.compaction.threshold_tokens` are both `Option`, both **unset** in `default.config.yaml`, and the only remaining reducer is the user typing `/compact`. The reason is the **prompt cache**: every provider that caches (Anthropic breakpoints, OpenAI automatic prefix caching) keys on the longest common *prefix*, so anything that rewrites history mid-conversation costs a full miss on the next request.
+
+The two guards are not equally bad at that, and the difference is why one is merely off and the other is close to a trap. `max_history_messages` is a **sliding tail window** (`agent_loop::projection::window` — `drain(..len - max)`): past the cap it drops from the head on *every* turn, so it is a cache miss *per request*, forever, and it drops messages with **no summary standing in for them** — silent amnesia. Compaction rewrites the prefix **once per compaction** and leaves a summary behind. So the previous default — window on, compaction off — was the worse of the two in both dimensions, and the window's own doc-comment already said the two were mutually exclusive.
+
+Three consequences worth not re-deriving:
+
+- **The compactor is built unconditionally**, in both `bundles.rs` and `user_context.rs`. It used to be `Option<Arc<ContextCompactor>>`, keyed on the config section existing — which meant that commenting out `compaction:` also silently disabled **manual** `/compact` (`force_compact` returned `Ok(false)` and the chat answered "compaction disabled"). Manual compaction is a command a user types; it must not depend on an admin having filled in a token threshold. `try_compact` early-returns on `threshold_tokens: None`; `force_compact` deliberately does not consult it — the human *is* the trigger.
+- **The projection yields to the *automatic* pass, not to the compactor's existence**: `LoopConfig.auto_compaction_enabled` (`= ContextCompactor::auto_enabled()`), so a configured message cap is not silently voided by the mere availability of `/compact`. Expressed as `max_history_messages.filter(|_| !auto_compaction_enabled)` in `projection_cfg.rs`.
+- **`CompactionConfig`'s `Default` is hand-written**, same trap as `RoleAttrs`: a derived one gives `keep_recent: 0`, which would compact away every recent message on any box omitting the section — now the shipped default.
+
+The future automatic pass should trigger off the **resolved model's own context window**, not a hand-tuned `threshold_tokens` that has no idea which model is answering.
+
 ## Approval gate
 
 The rule engine `ApprovalManager::check` returns `Allow`/`Deny`/`Require` per tool call (default rules seeded on first boot; the catch-all `* require @999999` gates anything not explicitly allowed — e.g. `execute_cmd`, `execute_task`, writes outside whitelisted paths). It is wired to the loop as `loop_adapters/gate.rs::ApprovalGate` (`agent_loop::gate::Gate`). A `Require` registers a `oneshot` in the in-memory `pending` map keyed by `request_id` and emits an approval event over WS.