feat(memory): dual-pool memory namespace, FTS search, and prompt injection

Add a virtual memory namespace backed by SQLite, surfaced through the
fs-tools, with private (per-user) and shared (system) stores.

Storage
- `memory_docs` owner table + external-content FTS5 index with sync triggers.
- `db/memory_docs.rs` accessor: get / upsert / list / search (bm25+snippet) / delete.

Routing (tools/fs)
- `classify_memory` splits paths on the raw first component; `..` clamps inside
  the store, never escaping to disk.
- read/write/list/edit/insert/replace/search_file route `user-memory/` to the
  owner pool and `shared-memory/` to the system pool (a singleton captured in
  `register_all`); every other path stays on disk. Each tool extracts a pure
  transform shared between its disk and memory paths.
- New `memory_search` tool over the FTS index (scope private/shared/all),
  with a sanitised FTS5 query. grep_files stays disk-only.

Approval
- `user-memory/*` allow (read+write); `shared-memory/*` reads allow,
  writes require approval so the agent can't silently push one person's data
  into shared memory. `memory_search` allowed via a path-less rule.
- migrate away the old `memory/*` and blanket `shared-memory/*` rows.

Prompt injection
- `MessageBuilder::load_inject_memory` reads `user-memory/` (owner pool) and
  `shared-memory/` (system pool) inject entries from SQLite; disk paths
  unchanged. The system pool is threaded ChatSessionManager -> handler ->
  MessageBuilder.
- main and project-coordinator inject `user-memory/index.md` +
  `shared-memory/index.md`; common/memory.md rewritten for the two stores.

crates/skald-core/src/session/handler/mod.rs

@@ -262,6 +262,9 @@ impl ApprovalDecision {
 pub struct ChatSessionHandler {
     pub session_id:              i64,
     pub(super) db:               Arc<SqlitePool>,
+    /// The shared (`system.db`) pool. Owner-bound work uses `db`; this is only for
+    /// cross-owner reads, e.g. injecting `shared-memory/` notes into the prompt.
+    pub(super) shared_pool:      Arc<SqlitePool>,
     /// The authenticated user who owns this session. Threaded into `ChatOptions`
     /// so the telemetry metadata row in `system.db` carries `user_id`.
     pub(super) user_id:          String,
@@ -332,6 +335,7 @@ impl ChatSessionHandler {
     pub fn new(
         session_id:            i64,
         db:                    Arc<SqlitePool>,
+        shared_pool:           Arc<SqlitePool>,
         user_id:               String,
         llm_manager:           Arc<LlmManager>,
         max_history_messages:  usize,
@@ -357,6 +361,7 @@ impl ChatSessionHandler {
         Self {
             session_id,
             db,
+            shared_pool,
             user_id,
             llm_manager,
             max_history_messages,