feat(channel): THESEUS ledger sink — spoken and dashboard turns become durable receipts

[chazmaniandinkle]

What

Every conversational utterance through the channel client — mod3_speak with post_to_chat=True, mod3_dashboard_post as assistant — is also committed as a receipt into the book repo's conversations/inbox/, riding the existing ingest workflow into the THESEUS ledger. mod3's message store is RAM-only (2026-07-23 tomography, gap #6); this is the additive fix at the mouth. The daemon is untouched.

Design

• Receipts, never direct ledger writes — new-file-only, settled idempotently by id by the existing conversations-ingest workflow; no content contention with the seat or the ingest bot.
• Identity declared, never inferred — every turn carries origin=seat and a seat-* from (both self-echo suppression keys for the wake line), and the sink enforces the prefix itself.
• Speech never blocks on the ledger — fire-and-forget via asyncio.to_thread; failures log and swallow; kill switch MOD3_THESEUS_SINK=0.
• Interprocess flock per repo serializes write→add→commit→push across concurrent channel-client processes and both mouths (git's index.lock is fail-fast, not a queue); receipt ids carry an entropy suffix so same-millisecond writers can't collide; _cleanup drains in-flight sinks (bounded 15s) so a session's final utterance isn't cancelled mid-write.

Verification

• 17/17 tests (tests/test_theseus_sink.py — real bare-origin+clone git pairs, including a 6-thread concurrency test and the remote-moved rebase race; channel-client regression suite green).
• Live end-to-end twice against the real book repo: receipt committed → pushed → ingest settled the turn into ledger.json with origin=seat — and the wake-line watcher stayed silent on both commits (no mark ... seat echo).
• Three-lens adversarial review (concurrency / identity-membrane / failure-modes); both blockers and all should-fixes addressed in the hardening commit, except sink-outcome-invisible-to-caller, which is the intended fire-and-forget contract.