feat(session): archive compaction-dropped messages to disk (M1.4)

[wusijian007]

Compaction used to replace the dropped middle slice with an inline "[context compacted: N messages omitted]" notice and discard the originals. That made the operation lossy: there was no way to recover what got cut, and no way to audit a long-running session's compaction history.

Surface introduced:

• CompactOptions.archiveSink?: (omitted: readonly Message[]) => void on compactMessages. Synchronously hands the caller the original (unsnipped) dropped slice. Default is no-op, so headless query-loop retries on prompt_too_long stay unchanged.

• compactSessionRecord becomes async and accepts an optional archiver: (omitted, at) => Promise<string|undefined>. The returned path is stamped onto the resulting compact SessionEvent as archivePath. Omitting the archiver, or returning undefined, keeps archivePath unset for back-compat with older sessions.

• myagent compact <id> and the TUI /compact wire a default archiver that writes JSON to .myagent/artifacts/<sessionId>/compactions/<at>.json. The CLI prints the archive path on success. Filename uses at with : and . replaced by - so it round-trips on Windows.

• myagent resume <id> --show-compactions lists every compact event on the session with its archive path (or <not archived> for pre-M1.4 sessions / query-loop retries with no archiver).

Tests added in packages/core/test/security/compaction-archive.test.ts pin the new invariants (sink fires only on omission, archiver path round-trips, undefined archiver/return keeps archivePath unset). A CLI-level e2e test exercises the whole path: build a fixture session big enough to actually trigger omission, run myagent compact, read back the JSON archive, verify content is unsnipped, then run myagent resume --show-compactions and assert the archive path is listed. Catalog row added to packages/core/test/security/README.md; CLAUDE.md updated.