fix(desktop): refresh @ search when channel membership changes

[loganj]

🤖

Summary

The @ search list could keep old agent information after someone joined or left a channel from another client. Buzz now requests new agent information when it receives a channel membership update. You do not need to restart the app to request that refresh.

Buzz cancels older requests so their results cannot replace the new information. It combines updates that arrive within 200 milliseconds into one refresh. It ignores recently repeated updates and cancels queued work when you switch communities.

This is draft PR 2 of 7, based on #7190. It handles updates received from the server. Refreshes after your own create, Add, join, and leave actions follow in #7192. It does not change who can access a channel.

Related issue

Follow-up to #7122 and #7124. The shared base is 802ba6170e57ac71d1ecfd6cf6fb12f02ecf910c. It includes the required agent search, sending, spacing (#7128), and recipient identity (#7133) changes on main revision 114dbf745f1fe0bdbecb70e07a07f03c23687bdc. Review this PR against #7190, not against main. Separate forum and unrelated timeline work are excluded.

Testing

• This exact prefix 9164074d7be6608a8a83172f3a13b33782c51928 passed TypeScript and 61 focused tests in the packaging validation. Its code is unchanged since that run.
• The old published seven-slice source (before subsequent onboarding/fixture repairs) passed 165 affected Chromium browser cases, 161 focused tests and five ordinary Enter/code-fence/multiline/emoji controls, with no retries or failures. The final committed tree was checked against that tested source.
• The 6,122-test desktop suite passed before the final narrow shared-editor Enter change. That historical full-suite result is not a new execution of the repaired endpoint. Fresh build/type/style and differential file-size checks passed; a full repository just ci was not repeated for this continuation.
• Browser checks use simulated desktop services. They do not prove native/WebKit behavior, live-relay acceptance, agent response or atomic access-check/publication. The exact published-head CI result is recorded below; historical passes remain scoped to their original revisions.
• Immediate base: d0c3498409a8d25a9c4663547439312783aec96c. Head: 9164074d7be6608a8a83172f3a13b33782c51928. Footprint including tests: 226 additions + deletions. Fixed stack root: 802ba6170e57ac71d1ecfd6cf6fb12f02ecf910c.

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

• CI run completed successfully for 9164074d7be6608a8a83172f3a13b33782c51928: 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.
• GitHub also retains canceled duplicate same-head check records; the linked noncanceled run is successful.
• 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.