feat(search): search archived threads and messages

[coygeek]

Summary

Extend thread search so archived conversations can be found by title and message content, with archive state shown in results.

Problem to solve

The orchestration search query excludes archived threads (apps/server/src/orchestration/Layers/ProjectionSnapshotQuery.ts:1014-1065), so archiving removes useful history from search even though archived threads remain available in the archive view.

Proposed behavior

Add an opt-in archived scope or an All threads scope to the existing search. Results should identify the project, archive state, matching snippet, and timestamp and open the selected thread without silently unarchiving it.

Acceptance criteria

• Existing active-thread search remains unchanged by default.
• A user can include archived threads and message-content matches from web, desktop, and mobile.
• Results show archived status and project identity and open the correct thread after reconnect.
• Ranking, pagination, and snippets remain deterministic for the same query.
• Searching never mutates archive state.

Affected area

Search contracts and SQL projection, sidebar search controls, client-runtime result state, and remote/mobile thread navigation.

Non-goals

This does not change archive retention, bulk deletion, or provider history.

Alternatives considered

The archive list alone cannot find message content and requires manual paging through old threads.

Supporting context

The repository defines search input/result contracts in packages/contracts/src/orchestration.ts:2131-2150, while the SQL explicitly filters threads.archived_at IS NULL. Existing issue #11743 concerns sidebar search not searching message content; this proposal scopes the independent archived-search behavior and should be reconciled with that work.

Related report: sidebar message-content search #11743. This request specifically extends search to archived history; it does not assert that all existing active-thread search is absent.

Analysis basis: source inspection of T3 Code revision 9d4bb550a658. Statements about missing UI or contracts refer to that revision, not every provider-native capability or unmerged branch. This is a feature proposal based on source review, not a runtime bug reproduction.