Make screenshot capture atomic and serialized

[hamzamerzic]

Problem

The authenticated screenshot helper can reuse one browser profile across parallel agents in the same chat. Without transaction ownership, two captures can interleave navigation and viewport changes. A failed final attempt also writes directly to the stable output path, so it can replace the last trustworthy image with a partial file. Browser command failures are repeatedly suppressed, while the fake-browser tests pay production retry sleeps and assert implementation text rather than behavior.

Fix

• Serialize the complete capture transaction per browser profile with a bounded lock.
• Capture into a same-directory temporary PNG, validate it, then atomically publish it so failures preserve the last known-good output.
• Centralize the browser command and fatal-error boundaries while keeping the distinct semantic retry and readiness checks intact.
• Surface the last useful browser diagnostic, require exact numeric in-shell app ids, and remove one pass-through viewport helper.
• Make fake-browser tests hermetic and fast, and verify daemon identity behavior instead of shell source spelling.

Prior work

PR #321 established settled, correctly sized authenticated captures. This follow-up preserves those contracts and addresses the remaining output-publication, same-profile concurrency, diagnostic, and test-efficiency gaps.

Verification

• Upstream review worktree: 44 focused screenshot and browser-environment tests passed in 9.9 seconds.
• Shell syntax, Python compile, and canonical diff checks passed.
• Authenticated live capture produced a served 1,512×861 PNG with one settled chat surface; the browser session closed cleanly.