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/db/known_tools.rs

@@ -0,0 +1,94 @@
+//! `known_tools` — every tool ever offered to the LLM, captured at injection
+//! time by [`crate::tool_discovery::ToolDiscovery`].
+//!
+//! This is the drift-proof half of tool visibility: instead of maintaining a
+//! parallel list of "all tools", we record what is actually assembled into the
+//! LLM request (`AgentRunConfig::all_tool_defs`). The approval / Security-groups
+//! UI merges these rows so tools injected outside the `ToolRegistry` (interface
+//! tools, plugin tools, provider tools) can still be assigned a permission.
+
+use anyhow::Result;
+use serde::Serialize;
+use sqlx::SqlitePool;
+
+#[derive(Debug, Clone, Serialize)]
+pub struct KnownTool {
+    pub name:        String,
+    pub description: String,
+    /// JSON parameters schema as last seen, if any.
+    pub schema:      Option<String>,
+}
+
+/// Records (or refreshes) a tool by name. Idempotent: re-seeing a tool updates
+/// its description/schema and bumps `last_seen`.
+pub async fn upsert(
+    pool:        &SqlitePool,
+    name:        &str,
+    description: &str,
+    schema:      Option<&str>,
+) -> Result<()> {
+    sqlx::query(
+        "INSERT INTO known_tools (name, description, schema, first_seen, last_seen)
+         VALUES (?1, ?2, ?3, strftime('%s','now'), strftime('%s','now'))
+         ON CONFLICT(name) DO UPDATE SET
+             description = excluded.description,
+             schema      = excluded.schema,
+             last_seen   = excluded.last_seen",
+    )
+    .bind(name)
+    .bind(description)
+    .bind(schema)
+    .execute(pool)
+    .await?;
+    Ok(())
+}
+
+/// All recorded tools, sorted by name.
+pub async fn all(pool: &SqlitePool) -> Result<Vec<KnownTool>> {
+    let rows = sqlx::query_as::<_, (String, String, Option<String>)>(
+        "SELECT name, description, schema FROM known_tools ORDER BY name",
+    )
+    .fetch_all(pool)
+    .await?;
+    Ok(rows
+        .into_iter()
+        .map(|(name, description, schema)| KnownTool { name, description, schema })
+        .collect())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn temp_db_path(tag: &str) -> String {
+        let mut p = std::env::temp_dir();
+        let nanos = std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH).unwrap().as_nanos();
+        p.push(format!("skald-test-{tag}-{}-{nanos}.db", std::process::id()));
+        p.to_string_lossy().into_owned()
+    }
+
+    fn cleanup(path: &str) {
+        for suffix in ["", "-wal", "-shm"] {
+            let _ = std::fs::remove_file(format!("{path}{suffix}"));
+        }
+    }
+
+    #[tokio::test]
+    async fn upsert_is_idempotent_and_updates_metadata() {
+        let path = temp_db_path("known-tools");
+        let pool = crate::db::init_system_pool(&path).await.unwrap();
+
+        upsert(&pool, "send_voice_message", "v1", Some(r#"{"type":"object"}"#)).await.unwrap();
+        upsert(&pool, "send_voice_message", "v2", None).await.unwrap();
+
+        let rows = all(&pool).await.unwrap();
+        assert_eq!(rows.len(), 1, "same name must not create a second row");
+        assert_eq!(rows[0].name, "send_voice_message");
+        assert_eq!(rows[0].description, "v2", "description is refreshed on re-upsert");
+        assert_eq!(rows[0].schema, None, "schema is refreshed on re-upsert");
+
+        pool.close().await;
+        cleanup(&path);
+    }
+}