fix(desktop): check access again when you select or pin an @ mention

[loganj]

🤖

Summary

An @ choice could remain on screen after access changed or you left the channel. Selecting that old choice could insert a mention or enable automatic mentions. Buzz now checks the selected recipient again before it changes your draft.

The same check protects Automatically mention, the pin control that adds an agent to later messages in the conversation. If Buzz rejects a choice, it leaves your draft and automatic mentions unchanged. You can still use Don't automatically mention to remove an agent after access is denied.

For a team choice, Buzz checks every recipient. It rejects the whole choice if the recipients changed or a recipient is no longer selectable. A renamed label, duplicate entry, or changed order cannot substitute another recipient.

This is draft PR 4 of 7, based on #7192. It protects the existing controls. It does not add new access labels or change access rules. It does not fix every case where the text or text cursor changes before selection. Search completion changes follow in #7197.

Related issue

Continues #7192 and the agent search and mention work in #7122 and #7124.

Testing

• This slice was replayed without conflicts after the PR3 onboarding repair and, for PR6/7, the PR5 send-binding fixture repair. Its immediate-parent production diff is unchanged. Prior prefix validation remains historical evidence, not a fresh run on this head.
• Integrated onboarding repair: fresh build passed; 6/6 Welcome browser cases and 129/129 focused tests passed. The strengthened cached-roster regression fails against the old production build and passes against the repair, before exercising the original ambiguous-name/no-send/draft/exact-starter assertions.
• Final integrated production (unchanged by the additional test-only PR5 delta) passed 9/9 send-binding and agent-control browser cases, including Stop rejecting the foreign-channel result. Final TypeScript, four-file Biome and differential size checks passed. The earlier remote Stop timeout is not established as a picker defect; no control product/test change was made.
• The old published seven-slice tree aa90290870119e0bef2ba5ebcb6ce92c069db388 passed 165 affected Chromium cases, 161 focused tests and five bounded ordinary Enter/code-fence/multiline/emoji controls. Those results are reused for unaffected scope, not claimed as fresh whole-stack tests of this head. Historical 6,122 desktop tests predate the earlier Enter repair. Full repository just ci was not repeated.
• Browser checks use simulated desktop services: no native/WebKit, live-relay acceptance/delivery, or atomic access-check/publication claim. The exact-head CI result is recorded below; old-head onboarding/send-binding failures and canceled duplicate runs are not current-source failures.
• Immediate base: 07fbd2ccb354c9ab81b018811f9799cbb00901a7. Head: 1a8a942136a24e2c9dbba02c65f55e3485092826. Footprint including tests: 468 additions + deletions. Fixed stack root: 802ba6170e57ac71d1ecfd6cf6fb12f02ecf910c.

Exact-head CI follow-through (2026-09-02)

• CI run completed successfully for 1a8a942136a24e2c9dbba02c65f55e3485092826: 14 successful jobs, 10 path/condition-skipped jobs. This is the path-scoped remote workflow result, not a new claim that every repository test ran.
• Draft remains open; native/WebKit behavior, live-relay acceptance/delivery and atomic access-check/publication are not established by mock-browser checks.

Origin discussion: Buzz channel 8dcd7afb-0193-4a3e-8ae9-72838de89a1d; original source channel f7a9536a-1738-4bad-a888-b3ea25010ef1. Draft only; no merge authority.