fix: resolve history-shadow.ts race condition (#479)

[diberry]

Summary

\�ppendToHistory()\ in \history-shadow.ts\ uses a read → regex insert → write pattern with no synchronization. When two concurrent agents append to the same \history.md, the last write wins and the other agent's entry is silently lost.

Root Cause

The function reads the entire file, performs a regex-based section insertion, then writes the entire file back. Between the read and write, another caller can read the same (stale) version, compute its own update, and overwrite the first caller's changes.

Fix

1. In-process async mutex (\withFileLock) — serializes concurrent callers targeting the same file path. Uses a promise-chain pattern (zero dependencies). Different files run in parallel.
2. Atomic writes (\�tomicWriteFile) — writes to a temp file then renames, preventing concurrent readers from seeing partial content.

Zero new dependencies — pure Node.js built-ins only.

Tests

14 new tests in \ est/history-shadow.test.ts:

• Regression: single-caller append, sequential multi-append, section creation
• Race condition: 5 concurrent appends, 10 concurrent appends, cross-section concurrent appends — all verify zero data loss
• Cleanup: no lock files or temp files left behind
• API contract: create, read, delete, exists, error on missing shadow

All 14 pass. Existing agent tests (65 tests) unaffected.

Fixes #479

[bradygaster]

EECOM review: Race condition fix looks correct. Promise-chain mutex + atomic write via temp-file rename — solid pattern, zero new deps. 14 new tests cover regression, concurrency (5 and 10 parallel appends), cleanup, and API contract. Rebased on dev (includes #483 CI timeout fix).