fix(usage): serialize addSession to prevent lost updates on concurrent writes

[aakashsharma7]

Summary of changes

addSession() had a race when multiple sessions finished at the same time (for example parallel subagents). Each caller did a non-atomic read-modify-write on usage.json, so a later write could replace an earlier one using a stale baseline and drop usage data.

Changes

Atomic writes — write to ${filePath}.${uuid}.tmp, then fs.renameSync so concurrent readers never see empty or truncated JSON.
In-process lock— serialize read-modify-write with a Promise-chain lock (withUsageLock), same pattern as session-manager.ts.
addSession is async — now returns Promise<void> and runs inside withUsageLock.
Changeset + test — patch changeset, plus a concurrency regression test that fires several sessions with Promise.all.

Testing

• pnpm run test:ava source/usage/storage.spec.ts — 35 tests passed
• pnpm run test:types — 0 errors
• pnpm run test:knip— no unused exports
• pnpm run test:lint— Biome clean