Adds a user-triggered /compact action for Coder Agents chats: typing /compact in the composer (or picking it from the / trigger menu) summarizes the conversation so far to free up context window space.

How it works

• New POST /api/experimental/chats/{chat}/compact endpoint (owner-only, RBAC ActionUpdate, excluded from the public API reference via x-apidocgen skip). It marks the chat with a durable one-shot chats.compaction_requested_at signal and moves it waiting -> running via a new RequestCompaction state transition; no message row is inserted. AI Gateway attribution needs no per-request key: generation preparation resolves the owner's synthetic API key (feat: add synthetic gateway keys #27170) like any other turn.
• RequestCompaction hands off chat ownership (clears worker_id/runner_id) so a worker acquisition hint is published; since the transition changes no history, the previous runner could otherwise miss the request under reordered pubsub delivery.
• The background chat worker picks the chat up like any other turn. A pending manual request takes precedence over turn completion in the generation decision, and forces compaction even below the automatic threshold (and when compaction is disabled via threshold=100). The commit step consumes the request marker in the same transaction; any transition that ends the turn clears stale markers.
• The summary triplet reuses the automatic-compaction path, now tagged with a source (automatic | manual) that is plumbed through streamed progress parts, persisted tool JSON, and the UI label ("Summarized (manual)").
• Validation order: busy chats reject with 409 (state-machine conflict), empty/already-compacted chats with 409 "nothing to compact", archived chats with 400; the owner usage-limit check runs last so no-op requests surface the specific conflict instead of a limit error.
• Web UI: the / trigger menu now has a built-in "Commands" group listing /compact; submit intercepts exactly /compact and calls the endpoint instead of sending a message. A personal or workspace skill named compact takes precedence over the built-in command; while skill collisions are still resolving, an exact /compact submission is blocked with a retryable hint instead of leaking as message text. History and queued-message edits are never intercepted. After compaction, the context usage indicator resets to its unknown state until the next assistant response reports fresh usage, instead of showing the stale pre-compaction number.
• codersdk: ExperimentalClient.CompactChat.

Worker-path execution (rather than compacting synchronously in the handler) reuses the existing lock fencing, live "Summarizing..." streaming, retry accounting, restart resilience, and debug-run observability. Rationale documented in coderd/x/chatd/ARCHITECTURE.md.

Testing

• State machine: transition-matrix coverage for RequestCompaction, marker lifecycle tests (carried by lease renewals/queue appends, cleared by terminal transitions, consumed by commit), ownership handoff + acquisition hint assertions.
• Worker: decision-ordering and forced-compaction unit tests; active-server end-to-end test (manual compact below threshold produces a source=manual summary, returns to waiting, no assistant follow-up; busy chat rejected).
• API: success, archived, non-owner, RBAC-denied, empty-chat, no-daemon cases; usage-limit ordering (at-limit owners still get state/nothing-to-compact conflicts for no-op requests, with marker rollback).
• Frontend: Storybook play tests for the Commands menu group, submit intercept, skill-name collision, queued-edit passthrough, and manual/automatic tool rendering; unit tests for command availability resolution and the post-compaction context usage reset.

This PR was created by Mux, an AI coding agent, working on Mike's behalf.