perf(state): merge FTS5 segments on VACUUM + add 'hermes sessions optimize'

The FTS5 indexes (messages_fts, messages_fts_trigram) grow as a series of
incremental b-tree segments — one per trigger-driven insert batch. SQLite's
automerge caps at ~16 segments, so a long-lived store keeps scanning many
segments per MATCH and never collapses them unless the special 'optimize'
command runs. Nothing in the codebase ever ran it: vacuum() only fired after
a prune that deleted rows, and even then never merged FTS segments.

Changes:
- SessionDB.optimize_fts(): merges each FTS5 index to a single segment,
  probing for the (optional/lazy) trigram table first so it is safe to call
  unconditionally. Layout-only — search results and snippet() are unchanged.
- vacuum() now calls optimize_fts() before VACUUM so freed index pages are
  returned to the OS in the same pass.
- 'hermes sessions optimize' CLI subcommand for on-demand reclamation +
  segment compaction (previously there was no way to compact the store
  without a prune deleting rows), with before/after size reporting.

Benchmark (8000 msgs, fragmented to 8 segments/index):
- segments 8 -> 1 on both indexes
- porter MATCH 5.5x faster (0.449 -> 0.081 ms/q)
- trigram MATCH 3.0x faster (0.632 -> 0.207 ms/q)
- 8000 matches before == 8000 after, identical row ids (no functional change)

Orthogonal to the structural FTS-size PRs (#20239 external-content,
#27770 optional trigram) — segment merge helps regardless of those.

Tests: TestOptimizeFts covers index count, search+snippet preservation,
missing-trigram path, and idempotency. Full test_hermes_state.py green (227).

tests/test_hermes_state.py

@@ -2676,6 +2676,64 @@ class TestVacuum:
         db.vacuum()
 
 
+class TestOptimizeFts:
+    def test_optimize_returns_index_count(self, db):
+        """A fresh DB has both FTS indexes; optimize merges both."""
+        db.create_session(session_id="s1", source="cli")
+        db.append_message(session_id="s1", role="user", content="hello world")
+        assert db.optimize_fts() == 2
+
+    def test_optimize_preserves_search_and_snippet(self, db):
+        """Optimize is layout-only: MATCH results + snippets are unchanged."""
+        db.create_session(session_id="s1", source="cli")
+        for i in range(50):
+            db.append_message(
+                session_id="s1",
+                role="user",
+                content=f"needle alpha bravo charlie message {i}",
+            )
+        before = db.search_messages("needle")
+        n = db.optimize_fts()
+        assert n == 2
+        after = db.search_messages("needle")
+        assert len(after) == len(before)
+        assert len(after) > 0
+        # Snippet must still be populated (would be empty/None if the FTS
+        # content shadow were lost during optimize).
+        assert all(row.get("snippet") for row in after)
+        # IDs and snippets are identical before/after — pure layout change.
+        assert [r["id"] for r in after] == [r["id"] for r in before]
+        assert [r["snippet"] for r in after] == [r["snippet"] for r in before]
+
+    def test_optimize_skips_missing_trigram_table(self, db):
+        """When the trigram index is absent, optimize handles only the porter
+        index and does not raise."""
+        db.create_session(session_id="s1", source="cli")
+        db.append_message(session_id="s1", role="user", content="hello")
+        # Drop the trigram table + triggers to simulate a disabled/absent index.
+        with db._lock:
+            for trig in (
+                "messages_fts_trigram_insert",
+                "messages_fts_trigram_delete",
+                "messages_fts_trigram_update",
+            ):
+                db._conn.execute(f"DROP TRIGGER IF EXISTS {trig}")
+            db._conn.execute("DROP TABLE IF EXISTS messages_fts_trigram")
+        assert db._fts_table_exists("messages_fts_trigram") is False
+        assert db._fts_table_exists("messages_fts") is True
+        # Only the porter index remains -> 1 optimized, no error.
+        assert db.optimize_fts() == 1
+
+    def test_optimize_idempotent(self, db):
+        """Running optimize twice is safe (second pass is a no-op merge)."""
+        db.create_session(session_id="s1", source="cli")
+        db.append_message(session_id="s1", role="user", content="repeat me")
+        assert db.optimize_fts() == 2
+        assert db.optimize_fts() == 2
+        # Search still works after repeated optimization.
+        assert len(db.search_messages("repeat")) == 1
+
+
 class TestAutoMaintenance:
     def _make_old_ended(self, db, sid: str, days_old: int = 100):
         """Create a session that is ended and was started `days_old` days ago."""