Summary

Today `/v1/threads?include_archived=true` returns active + archived. Whalescale's "Archived chats" settings page needs ARCHIVED-ONLY. Currently has to fetch everything and filter client-side.

Proposal

Add either:

• (a) a new query param: `/v1/threads?archived_only=true`, or
• (b) a new endpoint: `GET /v1/threads/archived`

Prefer (a) — extends the existing handler, no new route.

Tracks

• whalescale#260 — Settings → Archived chats (no list-archived endpoint)
• Umbrella: whalescale#228

Acceptance

• New filter implemented in `list_threads` / `list_threads_summary` in `runtime_api.rs`.
• `docs/RUNTIME_API.md` updated.
• Whalescale UI confirms it can populate the archived-chats list without client-side filtering.