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/projects.rs

@@ -0,0 +1,103 @@
+use anyhow::Result;
+use sqlx::SqlitePool;
+
+#[derive(Debug, Clone, sqlx::FromRow)]
+pub struct Project {
+    pub id:          i64,
+    pub name:        String,
+    pub path:        String,
+    pub description: String,
+    pub run_context: Option<String>,
+    pub created_at:  String,
+    pub updated_at:  String,
+}
+
+const SELECT: &str =
+    "SELECT id, name, path, description, run_context, created_at, updated_at
+     FROM projects";
+
+pub async fn list(pool: &SqlitePool) -> Result<Vec<Project>> {
+    let rows = sqlx::query_as::<_, Project>(sqlx::AssertSqlSafe(format!(
+        "{SELECT} ORDER BY updated_at DESC"
+    )))
+    .fetch_all(pool)
+    .await?;
+    Ok(rows)
+}
+
+pub async fn get(pool: &SqlitePool, id: i64) -> Result<Option<Project>> {
+    let row = sqlx::query_as::<_, Project>(sqlx::AssertSqlSafe(format!("{SELECT} WHERE id = ?")))
+        .bind(id)
+        .fetch_optional(pool)
+        .await?;
+    Ok(row)
+}
+
+pub async fn create(
+    pool:        &SqlitePool,
+    name:        &str,
+    path:        &str,
+    description: &str,
+    run_context: Option<&str>,
+) -> Result<Project> {
+    let id = sqlx::query(
+        "INSERT INTO projects (name, path, description, run_context)
+         VALUES (?, ?, ?, ?)",
+    )
+    .bind(name)
+    .bind(path)
+    .bind(description)
+    .bind(run_context)
+    .execute(pool)
+    .await?
+    .last_insert_rowid();
+
+    let row = sqlx::query_as::<_, Project>(sqlx::AssertSqlSafe(format!("{SELECT} WHERE id = ?")))
+        .bind(id)
+        .fetch_one(pool)
+        .await?;
+    Ok(row)
+}
+
+pub async fn update(
+    pool:        &SqlitePool,
+    id:          i64,
+    name:        &str,
+    path:        &str,
+    description: &str,
+    run_context: Option<&str>,
+) -> Result<bool> {
+    let n = sqlx::query(
+        "UPDATE projects
+         SET name = ?, path = ?, description = ?, run_context = ?,
+             updated_at = datetime('now')
+         WHERE id = ?",
+    )
+    .bind(name)
+    .bind(path)
+    .bind(description)
+    .bind(run_context)
+    .bind(id)
+    .execute(pool)
+    .await?
+    .rows_affected();
+    Ok(n > 0)
+}
+
+/// Touch updated_at — called after every ticket operation so ordering by recency works.
+pub async fn touch(pool: &SqlitePool, id: i64) -> Result<()> {
+    sqlx::query("UPDATE projects SET updated_at = datetime('now') WHERE id = ?")
+        .bind(id)
+        .execute(pool)
+        .await?;
+    Ok(())
+}
+
+pub async fn delete(pool: &SqlitePool, id: i64) -> Result<bool> {
+    let n = sqlx::query("DELETE FROM projects WHERE id = ?")
+        .bind(id)
+        .execute(pool)
+        .await?
+        .rows_affected();
+    Ok(n > 0)
+}