feat(dev): tee stdout/stderr to logs/dev-YYYY-MM-DD.log

[icebear0828]

Summary

• Dev mode auto-tees process.stdout / process.stderr to logs/dev-YYYY-MM-DD.log. Patches the write functions at fd-level so console.log, console.warn, process.stdout.write, and the existing structured log.* helper all land in the file.
• Skipped in production (NODE_ENV=production — log shippers handle persistence there) and under Vitest (VITEST / NODE_ENV=test); manual opt-out via CODEX_PROXY_FILE_LOG=0.
• Motivation: dev logs previously lived only in the tsx watch terminal scrollback. Closing the pane or scrolling past the buffer lost them, leaving no audit trail when an upstream 422 (e.g. No tool output found for function call call_xxx) hit. We want every dev session's full stdout/stderr on disk by default.

Implementation

• src/utils/log-file.ts — installFileLogger({ dir, filename? }) opens an append fd with openSync, wraps process.stdout.write / process.stderr.write to call writeSync(fd, ...) first then forward to the original, returns uninstall() that restores the exact original function reference (not a .bind clone, so identity comparisons hold).
• src/utils/install-dev-logger.ts — side-effect import that gates on NODE_ENV / VITEST / CODEX_PROXY_FILE_LOG and installs into ${cwd}/logs/. Errors are swallowed to stderr only — file logging must never break the process.
• src/index.ts — added as the very first import so startup logs ([Init], [Config], [ModelStore], etc.) get captured too.
• logs/ is already covered by the root *.log gitignore.

Test plan

• npx vitest run tests/unit/utils/log-file.test.ts — 7 cases covering stdout/stderr tee, nested directory creation, uninstall restoration, append-mode persistence across reopens, default dev-YYYY-MM-DD.log filename, boolean return value preservation.
• npx vitest run tests/unit/ — all 1490 unit tests still pass.
• npx tsc --noEmit — clean.
• Live verified: tsx watch auto-reloaded, logs/dev-2026-05-06.log materialized with full startup banner + subsequent request logs.

Out of scope

• Log rotation / retention. Files accumulate one per day; cleanup is manual for now.
• Capturing in production. Log aggregators consume the JSON stdout there directly.