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/lib.rs

@@ -0,0 +1,47 @@
+//! The headless Skald core: storage, identity, LLM stack, tools, sessions.
+//!
+//! Nothing here knows what runs it. The process shell — HTTP server, desktop
+//! webview, setup wizard — lives in the crates that depend on this one. Concrete
+//! plugins are never named: `plugin::PluginManager` only ever sees
+//! `Arc<dyn Plugin>`, constructed by the consumer and handed to `Skald::new`.
+
+pub mod boot;
+pub mod config;
+pub mod config_store;
+pub mod skald;
+pub mod agents;
+pub mod approval;
+pub mod chat_event_bus;
+pub mod chat_hub;
+pub mod chatbot;
+pub mod clarification;
+pub mod command;
+pub mod compactor;
+pub mod crypto;
+pub mod elicitation;
+pub mod cron;
+pub mod db;
+pub mod events;
+pub mod image_generate;
+pub mod inbox;
+pub mod latex;
+pub mod llm;
+pub mod location;
+pub mod memory;
+pub mod mcp;
+pub mod notification;
+pub mod pending_registry;
+pub mod plugin;
+pub mod projects;
+pub mod provider;
+pub mod run_context;
+pub mod secrets;
+pub mod service_manager;
+pub mod session;
+pub mod tic;
+pub mod tool_catalog;
+pub mod tool_discovery;
+pub mod tools;
+pub mod transcribe;
+pub mod tts;
+pub mod users;