tooling(backfill): raise MAX_MESSAGES cap, surface truncation

[heavygee]

Summary

The old hardcoded MAX_MESSAGES = 2000 silently dropped the tail of any transcript with more than 2k records. Hit this concretely on a 2974-line chat (jessica-story founding chat): 974 lines lost with zero indication in either the script output or the wrapper's report.

Changes

• Default cap raised to 50_000 (covers all real chats sampled; 30MB jsonl import is well within memory budget for a one-time per-session ingest).
• Override via --max-messages <N> CLI arg on both backfill-agent-transcript.ts and attach-agent-chat.ts (forwarded), and via HAPI_BACKFILL_MAX_MESSAGES env.
• BackfillResult now returns { rawTranscriptLines, maxMessagesApplied, truncated } so callers can detect drops without re-parsing.
• backfill-agent-transcript.ts emits a console.warn line when truncating.
• attach-agent-chat.ts surfaces a prominent BACKFILL TRUNCATED warning at the end of the report (after the JSON dump) so the operator sees it even if it would otherwise be buried mid-output.
• New countTranscriptRecords() helper exported for callers that want the raw count without paying for full parsing.

Why this matters for #34 (ACP migration)

A lossy backfill produces a HAPI session whose messages table has incomplete scrollback. If you then migrate that session to ACP via #34's transplant, the in-store store.db content is unchanged (so the agent's brain is fine), but the HAPI hub/web UI scrollback is missing the tail. Reviewers can't audit what the agent did before migration. This fix makes backfill non-lossy by default so post-migration audit is trustworthy.

Test plan

• New tests: backfillSessionMessages flags truncated=true when transcript exceeds --max-messages cap, with correct rawTranscriptLines / maxMessagesApplied.
• New tests: truncated=false on a single-line transcript.
• Existing 8 tests still pass (10 total in suite).
• Cross-reviewer: confirm the console.warn writes to stderr and doesn't break stdout-as-JSON parsing for any wrapper script that pipes through jq.

Companion branch

tooling/legacy-chat-acp-alignment (#36) — adds the post-attach ACP migration hint and commits the operator-authored hapi-resurrect-session.sh. Together these two branches close the find→attach→backfill→resurrect→migrate loop for #34.

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