feat(state): honor HERMES_DISABLE_FTS_TRIGRAM to skip the trigram FTS index

[DayanaLorza]

What & why

The trigram FTS5 table (messages_fts_trigram) mirrors every message into a
second full-text index to support CJK substring search. It roughly
doubles FTS storage — on a real install it was ~70 MB, about half of a
137 MB state.db.

The codebase already documents a HERMES_DISABLE_FTS_TRIGRAM switch (see
optimize_fts's docstring) and the search/optimize paths already probe for the
table's existence — but the flag was never actually read. The trigram
table was created and rebuilt on every startup regardless, so the documented
opt-out did nothing.

This PR wires the flag for real, for deployments that never search CJK text and
want the storage back.

Changes

• _fts_trigram_enabled() reads HERMES_DISABLE_FTS_TRIGRAM via the shared
  env_bool helper.
• On open, when disabled: skip creating the trigram table and drop it (plus
  its triggers) if a previous run or the v10/v11 migrations created it, so the
  storage is reclaimed on the next VACUUM. The expected-trigger set used for
  repair detection is narrowed to the main triggers so no spurious rebuild fires.
• Re-enabling (unset the var) rebuilds and backfills the index from existing rows.
• Graceful-degradation fix: a 3+ CJK-character query previously used only
  the trigram MATCH path and returned empty when the table was absent. The
  existing LIKE block is extracted into _cjk_like_search() and reused as the
  fallback, so CJK search degrades to LIKE whenever the trigram index is missing
  (disabled, mid-backfill, or failed creation) instead of silently returning
  nothing. English/keyword search is unaffected throughout.
• Document the variable in reference/environment-variables.md.

How to test

# default: trigram present
python -c "from pathlib import Path; from hermes_state import SessionDB; \
  db=SessionDB(db_path=Path('a.db')); print(db._fts_table_exists('messages_fts_trigram'))"  # True

# disabled: trigram absent, English + CJK (via LIKE) still work
HERMES_DISABLE_FTS_TRIGRAM=1 python -c "...same... "  # False

Automated: TestDisableFtsTrigram covers default-on, disabled-fresh, CJK LIKE
fallback, drop-on-open, and re-enable-rebuild.

Test results / platforms

• pytest tests/test_hermes_state.py — 276 passed (271 existing + 5 new), no regressions
• ruff check hermes_state.py — clean
• Tested on Linux (Python 3.11)

[alt-glitch]

Duplicate of #27770 — same feature: wiring the documented HERMES_DISABLE_FTS_TRIGRAM env var in hermes_state.py to skip the CJK trigram FTS5 index (also overlaps #42190). Addresses #22478.

[teknium1]

Thanks for addressing a real state.db storage concern. This automated hermes-sweeper review is closing the PR under the standing configuration policy.

• The PR’s public mechanism is the non-secret behavioral flag HERMES_DISABLE_FTS_TRIGRAM (commit d32698aab615), including documentation in website/docs/reference/environment-variables.md.
• AGENTS.md:102-107 reserves .env / HERMES_* variables for credentials and requires behavioral settings and feature flags to be exposed through config.yaml.
• The underlying feature gap remains real: current hermes_state.py:1661-1675 still creates and maintains the trigram index unconditionally. This close is about the configuration mechanism, not the reported storage cost.
• The duplicate discussion on feat(state): make trigram FTS5 index optional #27770 records the same direction: re-scope this as a config.yaml setting and carry the current migration, trigger-repair, cleanup/re-enable, and CJK fallback behavior forward.

Please feel free to re-submit a focused config.yaml-based opt-out. The existing CJK fallback at hermes_state.py:4899-4956 and FTS maintenance behavior at hermes_state.py:1465-1555 / 1657-1678 are the current paths it should cover.

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Closed as not-planned per standing maintainer policy (env-var-for-config). This is an automated hermes-sweeper review.

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Closed as not-planned per standing maintainer policy (env-var-for-config). This is a design-direction decision, not a code-quality judgment — see the Contribution Rubric in AGENTS.md for what the project is looking for. If you believe this policy was misapplied to your change, comment here and a maintainer will take a look.