feat(users): UserManager with per-user SQLCipher, and extract skald-core crate

Two changes developed together in one session; they share the same module
structure (db/mod.rs, the core lib root) and only compile together, so they
land as one commit.

## UserManager + per-user encryption (§9/§11)

New `users::UserManager`: owns the system.db pool plus a map
`userid -> SqlitePool` of unlocked databases. The pool *is* the unlock token —
its connect options carry the DEK as SQLCipher's raw key, so an open pool means
the key is in RAM until restart and dropping it re-locks (§9). Knows nothing
about cookies.

New `crypto` module: envelope encryption. A random 256-bit DEK encrypts
`{userid}.db`; `users.database_password` holds it sealed with AES-256-GCM under
`Argon2id(password, salt)`. The AEAD tag is the password verifier — one
derivation both authenticates and yields the key, so encrypted users store no
second hash. Cleartext users store the Argon2id output directly, compared in
constant time. Argon2 runs in spawn_blocking behind a 2-permit semaphore
(256 MiB per derivation).

- SQLCipher via `libsqlite3-sys` `bundled-sqlcipher-vendored-openssl`, pinned
  <0.38 so it unifies with the one sqlx-sqlite links (a newer copy would apply
  the feature to a SQLite sqlx never uses). OpenSSL is vendored and static, so
  the binary stays self-contained.
- Schema split into `create_registry_tables` (instance-wide, no user key) and
  `create_owner_tables` (one owner's content, identical in every file). No FK in
  the owner bucket may reach the registry — enforced by a standalone test.
  Dropped `chat_history.model_db_id` (write-only, and the only registry-crossing
  key); moved `projects`/`project_tickets` into the owner bucket.
- Provisioning invariant: the file is written before the row, deleted after it,
  so a crash leaves an orphan file, never a user without a database. `open_db`
  never creates: a missing file is an error, not a silent empty database.

Not consumed yet: no login, call sites still use the shared system.db pool.

## Extract crates/skald-core

The headless core moves out of `src/` into its own crate; `skald` (server) and
the coming `skald-setup` are shells around it. Two dependencies on the shell
were inverted rather than dragged along, so the core names neither Tauri nor any
concrete plugin:

- `Plugin::tools(self: Arc<Self>)` — plugins contribute tools through this hook
  (sibling of `http_router`), so the core no longer downcasts to
  `MobileConnectorPlugin`.
- `tools::restart::set_restart_handler` — the desktop shell installs its
  teardown-and-respawn; the core defaults to the supervisor exit code. The core
  loses its `desktop` feature.
- `boot`'s stdout formatter moves to the binary (`src/boot_format.rs`); the core
  only emits tracing events.

All 79 core tests pass; the binary boots and serves in a clean directory, and
the mobile-connector tools still register through the new hook.

crates/skald-core/src/llm/providers/mod.rs

@@ -0,0 +1,39 @@
+pub mod anthropic;
+pub mod deepseek;
+pub mod lm_studio;
+pub mod ollama;
+pub mod openai;
+pub mod openrouter;
+pub mod zai;
+
+// Re-export so existing code that uses `providers::ServiceType` / `providers::RemoteLlmModelInfo` keeps working.
+pub use crate::provider::ServiceType;
+pub use core_api::provider::RemoteLlmModelInfo;
+
+use core_api::provider::{ApiProvider, LlmModelRecord};
+
+/// Computes the `extra_params` an OpenAI-compatible client should be built with,
+/// given a model's stored `extra_params` and its selected reasoning value. The
+/// provider translates the reasoning value into a request fragment via
+/// `reasoning_request`; that fragment's top-level keys are merged over
+/// `extra_params` (reasoning wins on conflict). Returns `None` when neither is set.
+pub(crate) fn extra_with_reasoning(
+    provider: &dyn ApiProvider,
+    model:    &LlmModelRecord,
+) -> Option<serde_json::Value> {
+    let reasoning = model.reasoning.as_ref().and_then(|v| provider.reasoning_request(v));
+    match (model.extra_params.clone(), reasoning) {
+        (base, None)          => base,
+        (None, overlay)       => overlay,
+        (Some(mut base), Some(overlay)) => {
+            match (base.as_object_mut(), overlay.as_object()) {
+                (Some(b), Some(o)) => {
+                    for (k, v) in o { b.insert(k.clone(), v.clone()); }
+                    Some(base)
+                }
+                // Non-object base: the reasoning overlay takes precedence.
+                _ => Some(overlay),
+            }
+        }
+    }
+}