Add invalid-JSON test for append-run-log #481

Goal

Add a test (or small smoke script check) that scripts/append-run-log.mjs exits cleanly on invalid JSON.

Pain source: Community PR #474 replaced a stacktrace with a Usage message + exit 1. Lock that behaviour in.

Files

• Prefer a tiny test under scripts/ or document a one-liner in an existing script test if present
• Or add scripts/append-run-log.test.mjs if that matches repo style

Acceptance criteria

• Invalid second arg (not JSON) → exit code 1 and message mentions Usage / valid JSON
• Valid minimal JSON entry does not throw on parse (can mock/skip file write if needed)
• No secrets; keeps change small

Estimated time: ~20–30 minutes · label: tooling

Comment "I'll take this" to get assigned.