feat(agent-computer): report bounded per-Bot profile bytes

[yxflc11]

Problem and change

A client can stop/reset one Bot's profile but cannot measure it without mounting the computer's private volume. Add token-protected GET /computers/profile-usage, returning only {profileBytes: number | null} for the existing validated Bot identity. It never starts Chromium, creates a session/profile, reads file contents, or grants deletion authority.

The Linux implementation counts regular-file logical bytes through open directory descriptors. It skips symbolic links without following their targets (including Chromium singleton links), uses O_NOFOLLOW when opening every child directory, and refuses hardlinks, special files, changed directories, cross-device traversal and unsafe totals. It permits one traversal per process, caps entries at 10,000 and depth at 16, and checks a 2-second cooperative deadline around filesystem operations. A kernel-blocked filesystem call cannot be forcibly interrupted. Missing profiles return 0; refused/incomplete measurements and unsupported platforms return null. No profile paths or partial totals are returned.

The route is separate from unauthenticated /health, rejects query parameters and non-GET methods, and uses no new dependencies. agent-computer/PROFILE_USAGE.md documents the exact scope, including Linux-only descriptor traversal and the lack of an atomic live-filesystem snapshot.

Verification

• Actual Linux HTTP dispatcher, profile usage, Bot identity and authentication tests: 46 passed, 1 unsupported-platform case skipped, using Bun 1.4.2 in the existing isolated browser image with no network and disposable profiles.
• macOS Bun 1.3.14: the unsupported-platform case passed; 4 Linux traversal cases skipped as intended.
• HTTP coverage verifies authentication, invalid Bot/path rejection, method restrictions, single-Bot byte count and no browser startup.
• No personal profile, login, model or external browser request was used.

Reviewed against upstream commit cb5dc32a44517622c6db4e527e61d3abb389b43c.