refactor: replace new Promise with Promise.withResolvers per AGENTS.md

[oldschoola]

Summary

Replaces new Promise((resolve, reject) => ...) with Promise.withResolvers() in 4 files, per the AGENTS.md convention: "use Promise.withResolvers() instead of new Promise((resolve, reject) => ...)".

Files changed

• packages/ai/src/registry/oauth/callback-server.ts — #waitForCallback assigns resolve/reject to instance fields
• packages/ai/src/auth-storage.ts — raceUsageWithSignal wraps promise + abort signal
• packages/ai/src/auth-broker/remote-store.ts — #raceWithSignal wraps promise + abort signal
• packages/coding-agent/src/modes/print-mode.ts — stdout flush wrapper

Verification

• bun check passes
• 16 tests pass across auth-broker-refresher, auth-storage-usage-cache, callback-server-manual-input

[roboomp]

Thanks @oldschoola — clean, scoped follow-up to the Promise.withResolvers() convention in AGENTS.md. Rank p1: semantics are preserved across all four helpers (the abort-listener wiring in callback-server.ts, auth-storage.ts, and auth-broker/remote-store.ts is observably identical, and print-mode.ts's sync-throw path stays equivalent under the async wrapper). Affected tests (auth-broker-refresher, auth-storage-usage-cache, callback-server-manual-input, auth-broker-remote-store, remote-auth-store) pass locally.

One should-fix nit: both changelog entries are missing the external-contribution attribution ([#3336] + [@oldschoola]) called out in AGENTS.md. One nit on print-mode.ts documenting the (harmless) shift in where a synchronous stdout.write throw surfaces. Neither blocks merge.

[roboomp]

should-fix: per AGENTS.md ("External contributions: Added feature X ([#456](https://github.com/can1357/oh-my-pi/pull/456) by [@username](https://github.com/username))"), this entry should carry the PR + author attribution. Suggest appending ([#3336](https://github.com/can1357/oh-my-pi/pull/3336) by [@oldschoola](https://github.com/oldschoola)). Same for packages/coding-agent/CHANGELOG.md.

[roboomp]

nit: small semantic shift worth a glance — in the previous form, a synchronous throw from process.stdout.write(...) was caught by the Promise executor and surfaced as a rejection. After the refactor it propagates synchronously out of the call before await promise runs. Because runPrintMode is async the caller still observes a rejected promise, so the contract is unchanged — flagging only so reviewers don't have to re-verify. No change requested.

[oldschoola]

Both review comments addressed in the latest push:

1. CHANGELOG attribution: Added ([#3336](https://github.com/can1357/oh-my-pi/pull/3336) by [@oldschoola](https://github.com/oldschoola)) to both CHANGELOG entries.

2. Sync-throw safety: Wrapped process.stdout.write(...) in a try-catch so synchronous throws (e.g. destroyed stdout) are converted to rejections, matching the original new Promise executor behavior.