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/projects/mod.rs

@@ -0,0 +1,108 @@
+pub mod tickets;
+
+use std::sync::Arc;
+
+use anyhow::Result;
+use sqlx::SqlitePool;
+
+use crate::db::projects::{self, Project};
+use crate::run_context::RunContext;
+
+pub struct ProjectManager {
+    db: Arc<SqlitePool>,
+}
+
+impl ProjectManager {
+    pub fn new(db: Arc<SqlitePool>) -> Self {
+        Self { db }
+    }
+
+    pub async fn list(&self) -> Result<Vec<Project>> {
+        projects::list(&self.db).await
+    }
+
+    pub async fn get(&self, id: i64) -> Result<Option<Project>> {
+        projects::get(&self.db, id).await
+    }
+
+    pub async fn create(
+        &self,
+        name:        &str,
+        path:        &str,
+        description: &str,
+        run_context: Option<&RunContext>,
+    ) -> Result<Project> {
+        let rc_json = run_context.map(|rc| rc.to_db());
+        projects::create(&self.db, name, path, description, rc_json.as_deref()).await
+    }
+
+    pub async fn update(
+        &self,
+        id:          i64,
+        name:        &str,
+        path:        &str,
+        description: &str,
+        run_context: Option<&RunContext>,
+    ) -> Result<bool> {
+        let rc_json = run_context.map(|rc| rc.to_db());
+        projects::update(&self.db, id, name, path, description, rc_json.as_deref()).await
+    }
+
+    pub async fn delete(&self, id: i64) -> Result<bool> {
+        projects::delete(&self.db, id).await
+    }
+}
+
+/// Builds the runtime `RunContext` for working on `project`, layering project-runtime
+/// fields over an optional pre-resolved `base` RC (which carries static config set at
+/// creation time, e.g. `security_group`).
+///
+/// Runtime fields computed here:
+/// - `working_directory` — always set to `project.path`.
+/// - `allow_fs_writes`   — project tree + Skald's own `data/` directory.
+/// - `system_prompt`     — project-context fragments prepended before any stored ones.
+///
+/// Shared by `ProjectTicketManager::start` (background ticket jobs) and the interactive
+/// project-chat session provisioning, so both work with identical context.
+pub fn build_runtime_run_context(project: &Project, base: Option<RunContext>) -> RunContext {
+    let mut rc = base.unwrap_or_default();
+
+    // Working directory is always the project path, overwritten at build time.
+    rc.working_directory = Some(project.path.clone());
+
+    // Absolute path to Skald's own data directory (user personal data store).
+    let skald_data = std::env::current_dir()
+        .unwrap_or_default()
+        .join("data")
+        .to_string_lossy()
+        .into_owned();
+
+    // Grant write access to the project tree and Skald's data directory.
+    if !rc.allow_fs_writes.contains(&project.path) {
+        rc.allow_fs_writes.push(project.path.clone());
+    }
+    if !rc.allow_fs_writes.contains(&skald_data) {
+        rc.allow_fs_writes.push(skald_data.clone());
+    }
+
+    // Build runtime context fragments and prepend before any stored ones.
+    // Note: working directory is intentionally omitted here — the date/time/OS/WD
+    // tail block in MessageBuilder already reflects the effective WD from RunContext.
+    let project_header = if project.description.is_empty() {
+        format!("You are working on project \"{}\".", project.name)
+    } else {
+        format!("You are working on project \"{}\". Description: {}", project.name, project.description)
+    };
+    let mut injected = vec![
+        project_header,
+        format!(
+            "Personal user data is available at: {}. \
+             Consult it when the task requires knowledge about the user.",
+            skald_data
+        ),
+    ];
+    injected.extend(std::mem::take(&mut rc.system_prompt));
+    rc.system_prompt = injected;
+
+    rc
+}