runtime: add app-wide thread library

[gaelic-ghost]

Summary

• add CodexAppServer.makeLibrary() and the app-wide Library observable for stored thread lists
• publish unarchived, archived, cwd-grouped, selected, app-snapshot, and current Git branch state for GUI/CLI consumers
• record query descriptor and roadmap guidance for cwd-first listing, future repo-root grouping, and richer Git observables

Verification

• swift test --filter CodexAppServerLibraryTests
• swift test
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Enforce query limit on library snapshot reads

libraryThreadSnapshots(query:) applies archive/cwd/search filters but never applies query.limit, so makeLibrary(configuration:) can load and publish every matching thread in local history even when callers requested a bounded query. This breaks the expected behavior of ThreadListQD.limit for local-first startup and can significantly increase memory/UI work for users with large thread stores.

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