agent-loop: new crate — LLM loop kernel + Model clients (phase 0)

Extract the LLM agent loop into a standalone workspace crate with zero
deps on skald-core/core-api (blueprint project-loop.md, D13-D15):

- kernel: round loop, model fallback with rebuild, parallel tool fan-out
  (ordered id alloc / bounded concurrent exec / ordered record), streaming
  deltas drained before outcomes, sticky cancellation
- models: OpenAiModel/AnthropicModel/OllamaModel/LmStudioModel ported from
  llm-client onto the Model trait; ModelError carries the HTTP status;
  is_retriable default = the 401/403/404/422 rule
- DTL as crate protocol (ToolRendering Inline/DeferredToolReference/
  SystemToolBlock; Anthropic conversions + Kimi system+tools passthrough),
  host catalog behind ActivationSource/ToolActivator
- HistoryStore durability contract + InMemoryStore; LinearAssembler with
  well-formed projection (incl. DTL injection, summary, crash survivors)
- LoopManager singleton (broadcast bus + live registry), one live loop
  per conversation, orphan-marking on start_turn
- 32 tests green (kernel §13 suite, assembler DTL, SSE/Anthropic ports),
  clippy clean

crates/agent-loop/src/activation.rs

@@ -0,0 +1,112 @@
+//! Dynamic tool loading (DTL) — the wire PROTOCOL lives in the crate
+//! (blueprint D15), the catalog and persistence stay with the host.
+//!
+//! Three rendering modes ([`ToolRendering`]) decide how dynamically-activated
+//! tools reach the model without invalidating the prompt-cache prefix:
+//!
+//! - `Inline`: active tools go in the `tools` array (every activation changes
+//!   the array — no cache).
+//! - `DeferredToolReference`: all activatable tools are declared upfront with
+//!   `defer_loading: true`; an activation's tool result carries a
+//!   `_tool_references` marker the Anthropic client converts to
+//!   `tool_reference` blocks.
+//! - `SystemToolBlock`: activated tools never touch the `tools` array; a
+//!   `{role:"system", tools:[…]}` message is appended after the activation's
+//!   tool-result group (Kimi/Moonshot speaks this natively).
+
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use serde_json::{Value, json};
+
+use crate::ids::MessageId;
+use crate::tool::{Tool, ToolCtx, ToolFailure, ToolOutput};
+
+/// How dynamically-activated tools are rendered on the wire. On
+/// [`crate::model::ModelInfo`]; read by `ToolSet::defs` and assemblers,
+/// consumed by the shipped clients.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum ToolRendering {
+    /// Only the currently-active tools in the `tools` array.
+    #[default]
+    Inline,
+    /// Anthropic: all activatable tools `defer_loading: true` + tool_reference
+    /// blocks in activation results.
+    DeferredToolReference,
+    /// Kimi K3: `{role:"system", tools:[defs]}` appended after the activation
+    /// (append-only, cache-safe).
+    SystemToolBlock,
+}
+
+/// One activation: the defs of the groups activated at a given anchor message.
+#[derive(Debug, Clone)]
+pub struct Activation {
+    pub anchor: MessageId,
+    /// OpenAI-shaped tool defs of the groups activated at `anchor`.
+    pub defs:   Vec<Value>,
+}
+
+/// Catalog + persistence of activations — implemented by the host. Consulted
+/// by assemblers (injection) and by host `ToolSet`s (array rendering).
+#[async_trait]
+pub trait ActivationSource: Send + Sync {
+    /// The activations in force for a frame, ordered by anchor.
+    async fn activations(&self, frame: crate::ids::FrameId) -> crate::Result<Vec<Activation>>;
+}
+
+/// Backend of the shipped [`ActivateToolsTool`]: validates the groups, mutates
+/// the grants, persists the activation (anchored at the current message via
+/// `ctx`). Returns the confirmation text shown to the model.
+#[async_trait]
+pub trait ToolActivator: Send + Sync {
+    async fn activate(&self, groups: Vec<String>, ctx: &ToolCtx) -> Result<String, ToolFailure>;
+}
+
+/// The shipped `activate_tools` tool. To the kernel it's a tool like any
+/// other — the defs re-read at the next round makes the new grants visible.
+pub struct ActivateToolsTool {
+    activator: Arc<dyn ToolActivator>,
+}
+
+impl ActivateToolsTool {
+    pub fn new(activator: Arc<dyn ToolActivator>) -> Self { Self { activator } }
+}
+
+#[async_trait]
+impl Tool for ActivateToolsTool {
+    fn name(&self) -> &str { "activate_tools" }
+
+    fn definition(&self) -> Value {
+        json!({
+            "type": "function",
+            "function": {
+                "name": "activate_tools",
+                "description": "Load additional tool groups on demand. Activated tools \
+                                become available from the next step of this conversation.",
+                "parameters": {
+                    "type": "object",
+                    "properties": {
+                        "groups": {
+                            "type": "array",
+                            "items": { "type": "string" },
+                            "description": "Names of the tool groups to activate"
+                        }
+                    },
+                    "required": ["groups"]
+                }
+            }
+        })
+    }
+
+    async fn call(&self, args: Value, ctx: &ToolCtx) -> Result<ToolOutput, ToolFailure> {
+        let groups: Vec<String> = args["groups"]
+            .as_array()
+            .map(|a| a.iter().filter_map(|v| v.as_str().map(str::to_string)).collect())
+            .unwrap_or_default();
+        if groups.is_empty() {
+            return Err(ToolFailure::Failed("activate_tools: no groups given".into()));
+        }
+        let text = self.activator.activate(groups, ctx).await?;
+        Ok(ToolOutput::Text(text))
+    }
+}