feat(state): add config.yaml flag to disable trigram FTS index

[soe219]

Summary

Closes #55233. Adds an opt-in config.yaml switch — state.disable_fts_trigram (default false) — to skip the messages_fts_trigram FTS5 index. That index over-indexes every 3-character window and becomes the largest object in state.db (multiple GB on long-lived gateways) purely to serve CJK/substring search many deployments never use.

Default is unchanged behavior; existing users see no difference unless they opt in.

Addressing the prior reviews

This supersedes the closed #27770 / #42190 and folds in the reviewer feedback from the closed #57761:

• Config, not env (AGENTS.md). The user-facing knob is config.yaml: state.disable_fts_trigram, read via the same fast_safe_load + managed-scope-overlay pattern as _read_logging_config(). HERMES_DISABLE_FTS_TRIGRAM is retained only as an internal bridge (tests/managed deploys), not a documented switch — so .env stays credentials-only.
• No boot rebuild loop. _fts_trigger_count() / the startup repair check now expect the base-only trigger set when trigram is disabled. Previously a disabled index looked like "triggers missing" and forced a full FTS rebuild on every boot (the regression flagged on feat(state): opt-out flag for the trigram FTS index (HERMES_FTS_TRIGRAM=0) #57761). Covered by test_disabled_no_boot_rebuild_loop.

Behavior when enabled

• Startup and the legacy migration path skip creating messages_fts_trigram and its triggers.
• An already-present trigram index + triggers are dropped (_drop_fts_trigram), so a bloated state.db reclaims the space — pages return to the OS on the next VACUUM. Fully rebuildable from messages if re-enabled.
• Standard word/token search is unaffected (messages_fts); only CJK/substring queries fall back to LIKE.
• Also removes the misleading optimize_fts() docstring that referenced an unimplemented HERMES_DISABLE_FTS_TRIGRAM switch.

Tests

New tests/test_hermes_state_trigram_disable.py (6 cases): fresh-DB skip, no boot-rebuild loop, default keeps trigram, existing index dropped on opt-out, config.yaml read path (true/false/absent), env-bridge override. Existing tests/test_hermes_state.py: 382 passed, no regressions.

Reclaiming space on an already-bloated database

# config.yaml
state:
  disable_fts_trigram: true

Restart the gateway (drops the index), then sqlite3 state.db "VACUUM;" to return the freed pages to the OS.