fix(state): classify FTS-scoped corruption as fts_index, never whole-file damage

classify_persistence_error bucketed every _DB_CORRUPTION_MARKERS hit as "corrupt",
so an error SQLite itself scoped to the FTS5 index layer (SQLITE_CORRUPT_VTAB, or an
`fts5: corrupt structure record for table "messages_fts"` report) that escaped the
write path — the detach in _enter_fts_fail_open refused (generation/lock check),
or a read/search path with no fail-open at all — reached the turn boundary and the
gateway startup notice as structural corruption: the turn ended with `.recover` /
restore-backup advice on a file whose canonical tables were provably healthy.

One provenance rule, hermes_state_errors.is_fts_scoped_corruption_error, now feeds
both the write-repair gate (SessionDB._is_fts_write_corruption_error delegates to it,
so the gateway transcript retry inherits it) and the classifier: a known result code
outranks prose (only SQLITE_CORRUPT_VTAB is FTS-scoped; bare SQLITE_CORRUPT/NOTADB
and any contradictory code fail closed), and without a code the text must both carry
a corruption marker and name a messages_fts* object. The new "fts_index" cause
renders index-scoped guidance (doctor --fix / restart, do not run recovery) in the
turn explainer and the home-channel notice. The structural fail-close is untouched:
bare malformed / not-a-database still quarantine and still classify "corrupt".

Salvaged from PR #97843 (SulthanZahran1), trimmed: the quick_check-backed
"corrupt_unconfirmed" tier is dropped — on a live handle that just observed an
unscoped SQLITE_CORRUPT, PRAGMA quick_check on a damaged shadow b-tree raises rather
than reports on 3.53.1, so the probe could never downgrade the exact shape it was
built for, and a verdict that softens quarantine guidance on prose alone weakens the
fail-close. #97841 (Finn763) reached the same fts_index cause via text markers
only; its LIKE-degradation intent already lives in _search_messages_impl (_fts_stale).

Fixes #97794
Co-authored-by: finn763 <165816600+finn763@users.noreply.github.com>

agent/turn_explainers.py

@@ -151,6 +151,16 @@ _PERSISTENCE_CAUSE_EXPLANATIONS: Dict[str, str] = {
         "3. Restore from a backup in {backups_dir}/\n"
         "Then send your message again."
     ),
+    # SQLite scoped the corruption to the FTS index and the derived indexes could not be
+    # detached, so this write did not land; the message store itself is intact (#97794).
+    "fts_index": (
+        "the turn was stopped because the session search index (FTS5) "
+        "is corrupt and could not be detached, so this message was not "
+        "saved. The message store itself is not damaged: do not run "
+        "recovery tools or restore a backup. Run `hermes doctor --fix` "
+        "(or restart Hermes, which repairs the index on open), then "
+        "send your message again."
+    ),
     "disk": (
         "the turn was stopped because session storage could not "
         "be written (the transcript would have been lost on "