feat(brain): identity merge + claw merge + window attachment summaries (PR 3)

[topcoder1]

Summary

PR 3 of the Discord/Signal brain ingest series — finishes the chat-ingest UX layer with three sub-features:

1. Identity-merge engine (Tasks 1-2)

`mergeEntities(keptId, mergedId, opts)` atomically pivots all `ku_entities`, `entity_aliases`, and `entity_relationships` rows from loser → winner, writes a row to `entity_merge_log` with snapshot + evidence. Both entity rows survive (soft-delete via merge log). Rejects: self-merge, missing entities, type mismatch, re-merging an already-merged loser.

2. `claw merge` chat command (Tasks 3-7)

• New `EntityMergeRequestedEvent` type
• Signal + Discord channels detect `claw merge ` text trigger (with shared `parseMergeArgs` supporting bare words and double-quoted phrases)
• Brain-side handler resolves both handles by alias `field_value` or canonical name (case-insensitive), rejects ambiguity, calls `mergeEntities`
• Sends ack reply via optional `sendReply` callback (production wiring deferred to follow-up; merges still run end-to-end)
• Lifecycle wired into `startChatIngest`/`stopChatIngest`

3. Window attachment summaries (Tasks 8-9)

• `summarizeAttachment(att, opts)` — three tiers: vision-summary for images when `BRAIN_IMAGE_VISION=true` and `local_path` exists, else `[: ]` fallback, else `[attachment]`. Vision failures fall back silently.
• `window-flusher` now appends summary lines per attachment in the transcript so the LLM extractor sees their content. `flushOne`/`flushIdle`/`flushAll` became async; cap-flush, daily-check, ticker, shutdown use fire-and-forget `void flushOne(...)`.

Test plan

• `npm run build` clean
• All 2474 tests pass (263 files; 33 new tests for PR 3)
• Identity merge: 8 engine tests + 9 handler tests (including lifecycle)
• claw merge: 4 channel tests (signal + discord, both happy + arg-count rejection)
• Attachment summary: 7 unit tests + 1 window-flusher integration test
• Operator-run e2e: send `claw merge ` in a brain-opted chat, verify `entity_merge_log` row + ku_entities rebound
• Operator-run e2e: send a windowed message with image attachment, verify `signal_window` payload `transcript` field contains `[image: ]` line

Schema

No new migrations — leverages existing `entity_merge_log` table from PR 1.

Out of scope

• Auto-merge engine (deterministic / Splink) — `BRAIN_MERGE_AUTO_LOW_CONF_REJECT` env reserved but unused
• `claw unmerge` — `pre_merge_snapshot` is captured for future undo
• Production `sendReply` wiring (channel router access for ack replies in chat) — handler runs no-op reply today

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