fix: de-flake hook-bus observe-hook test (JSON EOF race)

[saucam]

Symptom

main CI (daemon job) is failing intermittently:

SyntaxError: JSON Parse error: Unexpected EOF: src/tests/hook-bus.test.ts:255

Root cause — a pre-existing race, newly exposed

The test "emit() fires observe hooks without blocking the caller" installs an observe hook cat > "${marker}", fires it, then await waitForFile(marker) and JSON.parse(readFileSync(marker)).

waitForFile only checked existsSync. But cat > creates the file empty and writes the JSON asynchronously — so waitForFile returned the instant the empty file appeared, and the read raced the write, hitting an empty (or partial) file → Unexpected EOF.

This race has always been there; it started tipping over in CI only because the integration-test files added in #165/#166 raised bun test's file count/concurrency and widened the window. Reproduced locally at ~1 failure in 5 runs.

Fix

Replace waitForFile with waitForJson<T>(): poll readFile + JSON.parse until the file holds valid JSON (or timeout). Closes both the empty-file and any partial-write race, and hands the parsed object straight back to the test.

Verification

• hook-bus.test.ts: 10/10 clean (was ~1/5 flaky).
• Full offline suite: 1347 pass / 9 skip / 0 fail. tsc + biome clean.

🤖 Generated with Claude Code

[github-actions]

🤖 Gemini code review

This PR successfully resolves a flaky test in hook-bus.test.ts where a file existence check (waitForFile) raced against the asynchronous completion of the file write (cat > marker). It replaces it with a robust waitForJson helper that polls until the file exists and contains valid JSON, or times out.

Findings: 🔴 0 · 🟠 0 · 🟡 0 · 🟢 0

---

_{Tokens spent · ⬆️ Input: 1,174 · ⬇️ Output: 81 · Σ Total: 2,395
Total may be higher due to thinking token counts.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@05b2b3b). Learn more about missing BASE report.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #168   +/-   ##
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  Coverage        ?   84.51%           
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  Files           ?      104           
  Lines           ?    18319           
  Branches        ?        0           
=======================================
  Hits            ?    15482           
  Misses          ?     2837           
  Partials        ?        0           

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