fix(state): consolidate gateway SessionDB writers via process-wide shared registry

A gateway process opened state.db from ~12 call sites, each minting its
own writer connection, self._lock, close-time WAL checkpoint, and
token-writer thread. With N independent writers on one WAL file, one
connection's close-time checkpoint could race another's growth — the
lost/reordered-page-write signature across 11+ incidents (#90837).

Adds hermes_state_registry.py: a process-wide, per-path, refcounted
shared registry owning the writer boundary.

- acquire(path): same resolved path returns the same instance (one
  writer connection, one lock, one token-writer thread) for every
  long-lived in-process caller (gateway runner, SessionStore, per-agent
  lazy recall, cron per-job, mirror, channel_directory, slash_commands,
  shutdown_flush, session_search, react_to_message, delegate, mcp_serve,
  auto_archive, tui_gateway).
- close() on a shared instance is a NO-OP — the registry owns the
  lifecycle, so one caller's close can never tear down a writer other
  callers still hold.
- Generation-aware retirement on inode change: a replaced state.db
  RETIRES the live generation (never lent again) but keeps it alive for
  existing holders; release is object-keyed so holders of the old
  generation drain it independently of the new one. The old
  generation's own write path still fails with the typed
  StateDbReplacedError (existing protection, unchanged).
- Replacement-open failure leaves NO registry entry for the path —
  the next acquire retries fresh, never hands out a closed stale object.
- All teardown runs OUTSIDE the registry lock: a final release's WAL
  checkpoint can never stall acquisition for every state.db.
- close_shared_session_dbs() at gateway shutdown drains every
  generation (live + retired) as the final safety net.

CLI one-shots, recovery flows, and read-only cross-profile opens keep
using SessionDB() directly with their own close() — only long-lived
in-process sites route through the registry.

References #90837 (root-cause tracker stays open: the #10 EOF signature
and the WAL-lifecycle A/B verdict remain under investigation there).

mcp_serve.py

@@ -75,8 +75,8 @@ def _get_sessions_dir() -> Path:
 def _get_session_db():
     """Get a SessionDB instance for reading message transcripts."""
     try:
-        from hermes_state import SessionDB
-        return SessionDB()
+        from hermes_state import get_shared_session_db
+        return get_shared_session_db()
     except Exception as e:
         logger.debug("SessionDB unavailable: %s", e)
         return None
@@ -93,7 +93,8 @@ def _load_session_messages(session_id: str):
         return None, f"Failed to read messages: {e}"
     finally:
         try:
-            db.close()
+            from hermes_state import release_or_close
+            release_or_close(db)
         except Exception:
             logger.debug("Failed to close MCP SessionDB", exc_info=True)