fix(desktop): show an agent in @ search after you add it to a channel

[loganj]

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Summary

Makes a newly added agent immediately findable with @ in the channel you added it to — and makes the agent you just created in the welcome flow mentionable right away.

• After Add members, the first @ search finds the new agent. Before, accepting an addition in the Add member dialog (or creating a channel) didn't refresh the member lists behind @ search, so the agent you just added was missing from the mention suggestions until something else reloaded them. Now those caches are invalidated as soon as the addition is accepted, so the next @ search in that channel finds the agent. Join and leave events refresh the same caches (that wiring is fix(desktop): refresh @ search when channel membership changes #7191).
• The welcome flow's new starter agent is mentionable right away. When onboarding provisions your new starter agent, the welcome composer's member roster refreshes, so @ finds the agent you just created on the first try — including choosing the exact agent when its name is ambiguous.

These refreshes feed the next time the picker opens; an already-open suggestion list is deliberately kept stable (that contract is #7197).

Part of eight mention-editor PRs: shared recovery prerequisite #7190 → {#7191 → #7192, #7196 → #7323 → #7197 → #7239 → #7240}. This PR builds directly on #7191, whose membership refresh scheduler it uses.

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 membershipMentionJourney.test.mjs cover the accepted channel-creation and Add-member refresh paths and the welcome-roster invalidation; membershipDirectorySync.test.mjs is extended here.
• Browser tests: mention-picker.spec.ts adds the journey — add the agent, then the first @ search finds it; onboarding.spec.ts covers the welcome-flow roster after starter provisioning, including choosing the exact newly created agent when its name collides. The fixture keeps both same-name agents and retains the fixed root's baseline ambiguous-submit regression: a typed colliding mention must fail visibly, keep the draft, and publish no event. The production fix for that visible failure is supplied by the shared fix(desktop): keep your draft editable when an @ mention cannot be sent #7190 prerequisite, not duplicated in this freshness delta.
• 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 34262720439 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.

Historical capture state (not these heads)

The following original captions/links are retained as historical capture evidence only; none is relabeled as this restack.

Before: Add members cannot find the agent

At the parent revision, 21349006, the app already has an old agent list. Creating a channel does not refresh that list. Add members finds no matching agent. This image does not show a successful Add.

After: Add succeeds and the first @ search finds the agent

At the earlier revision shown here, c0da14c8, the same search finds Fresh Scout. Add succeeds. The first @Fresh search shows Fresh Scout as a channel member. This PR does not include the later Member and Invite labels.

[loganj]

🤖

Before: preceding prefix cannot find the agent in Add members

Actual 21349006 build, warm-stale directory. Search is blocked; no successful Add is claimed.

After: Add succeeds and the first @ picker includes the agent

Actual c0da14c8 build, same fixture. Fresh Scout is present after Add, without a manual directory refresh.