YeonGyu-Kim d60697bb13 fix: guard session_ids with optional chaining to prevent crash
boulderState?.session_ids.includes() only guards boulderState, not
session_ids. If boulder.json is corrupted or missing the field,
session_ids is undefined and .includes() crashes silently, losing
subagent results.

Changes:
- readBoulderState: validate parsed JSON is object, default session_ids to []
- atlas hook line 427: boulderState?.session_ids?.includes
- atlas hook line 655: boulderState?.session_ids?.includes
- prometheus-md-only line 93: boulderState?.session_ids?.includes
- appendSessionId: guard with ?. and initialize to [] if missing

Fixes #1672
2026-02-11 13:27:18 +09:00
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