Revert thread recencyAt for sidebar ordering

[pakrym-oai]

Why

Revert #27910 to remove the newly introduced thread recencyAt persistence and API behavior from main.

What changed

This reverts commit fac3158c2a783095768076489815f361fa9b0db4, including the state migration, thread-store propagation, app-server API surface, generated schemas, and related tests.

Validation

Not run before opening; relying on CI for the initial fast signal.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve applied migration 38 compatibility

If any state DB has already recorded this reverted migration as version 38, deleting it leaves version 38 represented by the different 0038_external_agent_config_imports.sql file; the runtime migrator only ignores missing newer versions and still validates checksums for known versions, so those users will fail state DB migration on startup with a checksum mismatch instead of opening their existing sessions. Please keep a compatibility path for the old version-38 checksum (or otherwise repair/renumber the migration history) before removing this file.

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