feat(triage): add runtime validation for decision field in parseDecisions

[keinstn]

Summary

• Adds a VALID_DECISIONS constant and runtime check in parseDecisions (scripts/triage/parse.ts) that throws a descriptive error when decision is missing or not one of ready|not_ready|needs_clarification
• Follows the existing d.number error style
• Adds a matching test case in test/triage-parse.test.ts

Closes #168

Test plan

• npm run test — all 357 tests pass including new throws when decision.decision is not a valid value
• npm run typecheck — clean

🤖 Generated with Claude Code

[keinstn]

[Baton Reviewer] Review: pass
Managed by Baton; do not edit the marker line manually.

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All acceptance criteria are met:

• VALID_DECISIONS constant and runtime check added in parseDecisions (scripts/triage/parse.ts)
• Error thrown when decision is missing or not one of the three valid values, with descriptive message following existing style
• Test case added in test/triage-parse.test.ts mirroring the decision.number pattern
• CI: Lint, Typecheck, and Test (ubuntu-latest) all pass

The prior formatting finding (multi-line VALID_DECISIONS array) has been resolved in the second commit. PR is ready for human review.