feat: add manual chat compaction via /compact (#27081)

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 (#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.

coderd/database/dump.sql

@@ -2096,6 +2096,7 @@ CREATE TABLE chats (
     context_dirty_resources jsonb,
     context_error text DEFAULT ''::text NOT NULL,
     last_reasoning_effort chat_reasoning_effort,
+    compaction_requested_at timestamp with time zone,
     CONSTRAINT chat_acl_only_on_root_chats CHECK ((((parent_chat_id IS NULL) AND (root_chat_id IS NULL)) OR ((user_acl = '{}'::jsonb) AND (group_acl = '{}'::jsonb)))),
     CONSTRAINT chat_group_acl_not_null_jsonb CHECK (((group_acl IS NOT NULL) AND (jsonb_typeof(group_acl) = 'object'::text))),
     CONSTRAINT chat_user_acl_not_null_jsonb CHECK (((user_acl IS NOT NULL) AND (jsonb_typeof(user_acl) = 'object'::text))),
@@ -2119,6 +2120,8 @@ COMMENT ON COLUMN chats.context_error IS 'Snapshot-level error copied from the p
 
 COMMENT ON COLUMN chats.last_reasoning_effort IS 'Stores the most recent message effort once per-turn selection is wired.';
 
+COMMENT ON COLUMN chats.compaction_requested_at IS 'Set when the chat owner manually requests a context compaction. One-shot signal: consumed by the compaction commit and cleared whenever the chat leaves running.';
+
 CREATE TABLE users (
     id uuid NOT NULL,
     email text NOT NULL,
@@ -2214,7 +2217,8 @@ CREATE VIEW chats_expanded AS
     c.context_aggregate_hash,
     c.context_dirty_since,
     c.context_dirty_resources,
-    c.context_error
+    c.context_error,
+    c.compaction_requested_at
    FROM ((chats c
      LEFT JOIN chats root ON ((root.id = COALESCE(c.root_chat_id, c.parent_chat_id))))
      JOIN visible_users owner ON ((owner.id = c.owner_id)));