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).

hermes_cli/main.py

@@ -13390,6 +13390,11 @@ Examples:
         "--yes", "-y", action="store_true", help="Skip confirmation"
     )
 
+    sessions_subparsers.add_parser(
+        "optimize",
+        help="Reclaim disk space: merge FTS5 segments + VACUUM (no data change)",
+    )
+
     sessions_subparsers.add_parser("stats", help="Show session store statistics")
 
     sessions_rename = sessions_subparsers.add_parser(
@@ -13562,6 +13567,39 @@ Examples:
             relaunch(["--resume", selected_id])
             return  # won't reach here after execvp
 
+        elif action == "optimize":
+            db_path = db.db_path
+            before_mb = (
+                os.path.getsize(db_path) / (1024 * 1024)
+                if db_path.exists()
+                else 0.0
+            )
+            print("Optimizing session store (FTS merge + VACUUM)…")
+            try:
+                # vacuum() merges FTS5 segments (optimize_fts) then VACUUMs.
+                # Probe the index count first for the summary line.
+                n = sum(
+                    1
+                    for t in db._FTS_TABLES
+                    if db._fts_table_exists(t)
+                )
+                db.vacuum()
+            except Exception as e:
+                print(f"Error: optimization failed: {e}")
+                db.close()
+                return
+            after_mb = (
+                os.path.getsize(db_path) / (1024 * 1024)
+                if db_path.exists()
+                else 0.0
+            )
+            saved = before_mb - after_mb
+            print(f"Optimized {n} FTS index(es).")
+            print(
+                f"Database size: {before_mb:.1f} MB -> {after_mb:.1f} MB "
+                f"(reclaimed {saved:.1f} MB)"
+            )
+
         elif action == "stats":
             total = db.session_count()
             msgs = db.message_count()