What Problem This Solves

Fixes #99373. The ambient transcript watermark (#99306) survives session resets, but the transcript rows it vouches for do not: after any reset (manual, daily, delete), every prompt-facing window consumer kept filtering out messages at/before the watermark while those rows existed only in the archived transcript. Confirmed in production: a new session's transcript began exactly at watermark+1, with all older group messages permanently invisible — not in the window, not in the transcript, never compactable.

Why This Change Was Made

Watermark validity is a fact about a specific transcript (sessionId), not about the chat. The fix makes that structural:

• AmbientTranscriptWatermark now carries sessionId; writes stamp the entry's current sessionId.
• Reads (readAmbientTranscriptWatermark) treat a watermark whose sessionId differs from the entry's current sessionId as absent — self-invalidating on every reset path, with no per-reset-flow cleanup to forget. Legacy records without sessionId are ignored the same way (cache-like state: window backfills once; no doctor migration).
• Stale-hook guard: the room-event onMessagePersisted hook passes the prepared session id as expectedSessionId; if the session reset between the durable row write and the watermark advance, the advance is skipped instead of stamping a transcript that lacks the rows.

All consumers go through the read helper (Telegram session/prompt-context, live cache window, recovered threads, plugin-SDK facade), so they inherit the fix with no per-consumer changes.

User Impact

Ambient-mode group deployments no longer silently lose chat visibility across session resets: after a reset, the per-turn window backfills the persisted-but-archived messages (up to historyLimit) instead of hiding them forever. Steady-state dedupe behavior within a session is unchanged.

Evidence

• Regression tests reproduce the production scenario end-to-end: watermark written under one sessionId, entry reset to a new sessionId, Telegram structured prompt context backfills all rows (extensions/telegram/src/bot-message-context.prompt-context.test.ts); core read/write/legacy/stale-hook coverage in src/config/sessions/ambient-transcript-watermark.test.ts. Both fail-confirmed against the unfixed code (window collapsed to the unpersisted gap; legacy watermark still honored).
• Production LOC delta is 15 insertions / 2 deletions; the rest is tests.
• node scripts/run-vitest.mjs on touched tests, pnpm tsgo, pnpm check:test-types: exit 0.
• Blacksmith Testbox check:changed on the rebased branch: exit 0 (run tbx_01kwk866aemnttw2zjzgb8v8ek).

Diagnosis side-finding recorded on the issue: the observed intra-day daily reset is explained by process-UTC + DEFAULT_RESET_AT_HOUR=4 semantics (sessionStartedAt < resolveDailyResetAtMs), not a reset-policy bug.