fix(desktop): refresh @ search when channel membership changes - #7191

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Summary

Keeps the people and agents you can @ mention up to date as membership changes, without restarting or reopening anything.

• Membership events refresh the shared directory. When someone is added to or removed from a channel — by you or anyone else — the directory behind @ search is marked stale immediately and refreshed, so the new member can be found with @ the next time you type it. Before, the list could stay stale until something else happened to reload it.
• Bursts don't stampede. Several membership changes in quick succession coalesce into one refresh window instead of a pile-up of overlapping reads.
• A stale read can't come back and win. A directory read already in flight when the membership change lands is cancelled, so an older result can never arrive late and overwrite the fresh one.
• Switching communities starts clean. Queued refreshes are dropped when you switch community or signing identity, so work from the old community can't leak into the new one.

The refresh supplies up-to-date evidence for the picker only; it never grants permission or optimistically adds anyone to the list.

Part of eight mention-editor PRs: shared recovery prerequisite #7190 → {#7191 → #7192, #7196 → #7323 → #7197 → #7239 → #7240}. This PR builds on #7190 to preserve the baseline visible ambiguous-send failure behavior; membership freshness remains a separate feature unit.

Related issue

Continues the merged mention-editor work from #7124 (authorize remote mentions at publication) and #7128 (preserve spacing after multi-word mentions). No separate tracking issue for this slice.

Testing

• New unit tests membershipDirectorySync.test.mjs cover immediate staleness, coalescing, duplicate-event suppression, cancellation of in-flight reads, and reset on community or signing-identity switch.
• Rebased composition validation: TypeScript, whole-desktop Biome, repository file-size guard and E2E build pass. On the composed fix(desktop): show an agent in @ search after you add it to a channel #7192 tree, 88 focused scheduler/membership/send/helper tests and 11 accepted-write/roster edge probes pass; zero-retry browser checks pass for Create→Add→first @, baseline chat ambiguous-submit/error/draft/no-publication, and Welcome refreshed roster→preserved ambiguous negative→exact current starter sole-recipient completion. Unchanged freshness and restored Welcome source blobs were compared exactly with the prior reviewed source; no full-package rerun was needed for the mechanical replay. Exact published-head CI run 34262722726 completed successfully, including Desktop Core, all four smoke shards, relay-backed integration, and macOS/Windows builds. DCO, Semgrep and zizmor also pass. This is lane-local evidence, not a ready-to-merge or all-lanes-green claim.