feat: integrate agent context snapshots into chats (#26389)

Makes the chat context foundation from #26385 live. That PR added the
storage columns, writer queries, and a dormant
`agentapi.ContextDirtyMarker` trigger with no production callers; this
PR wires them together end to end.

When a workspace agent pushes a context snapshot, bound chats now
hydrate to that snapshot's hash, and a later push with a different hash
flips already-pinned chats to dirty (emitting a `context_dirty` watch
event after the transaction commits). Chat creation pins the agent's
latest snapshot when one already exists. The experimental chat API
exposes this as `Chat.Context` (`*ChatContext` with `dirty`,
`dirty_since`, `error`), and a new `PUT
/api/experimental/chats/{chat}/context` endpoint re-pins the agent's
latest snapshot and clears the dirty marker.

`context_dirty_resources` stays NULL (the resource-level diff is
deferred to the UI phase) and the live per-turn context pull is
unchanged.

The end-to-end test provisions a workspace agent via the echo
provisioner, connects it over the Agent API v2.10, and exercises the
full path: an initial push hydrates a bound chat (clean), a second push
with a different hash marks it dirty, the API reports the dirty state,
and the refresh endpoint clears it.

<details>
<summary>Decision log</summary>

- **API shape — sub-struct.** Dirty state is surfaced as
`codersdk.Chat.Context *ChatContext { Dirty bool; DirtySince *time.Time;
Error string }` rather than flat fields, matching the RFC's named
`ChatContext` type and leaving room for future fields (resource diff,
sources). `db2sdk.Chat` populates it when the chat is context-tracked
(`len(ContextAggregateHash) > 0`), dirty, or carries a snapshot error,
and leaves it nil (`omitempty`) otherwise. `Dirty` mirrors
`context_dirty_since` being set.
- **Marker wiring.** The chat daemon is injected directly as the
`agentapi.ContextDirtyMarker`. It is unconditionally constructed (only
its background worker is gated), so the marker is always non-nil and the
wiring matches every other `api.chatDaemon` call site. `agentapi` still
treats a nil marker as "chatd absent", so `PushContextState` stays a
pure write path for any future caller that does not wire chatd in.
- **Refresh is atomic.** `RefreshChatContext` reads the agent's latest
snapshot and re-pins the chat in one repeatable-read transaction, so a
concurrent push cannot land between the read and the write and leave the
chat pinned to a stale hash with the dirty marker cleared.
- **Hydrate + dirty run inside the push transaction.** The fan-out
shares the push's transaction so a concurrent refresh cannot interleave
with the version gate; `context_dirty` watch events publish only after
commit. The pinned hash on dirtied chats is intentionally left unchanged
— the refresh endpoint re-pins it.
- **Dirtied chats keep their pinned hash.** Drift is advisory: a dirty
chat stays usable, and refreshing is the only path that advances the
pinned hash.
- **Test binds `chats.agent_id` directly.** In production the binding is
set lazily during a chat turn (`chatd.persistBuildAgentBinding`); the
test sets it via `dbgen` so it exercises the context flow rather than
turn resolution.

Plan: `coderd/x/chatd` context integration + E2E (sub-struct API,
create-time + push-time hydration, refresh endpoint;
`context_dirty_resources` and the per-turn pull untouched).

</details>

🤖 Generated by Coder Agents on behalf of @kylecarbs

coderd/exp_chats.go

@@ -2618,6 +2618,39 @@ func (api *API) applyChatTitleUpdate(
 	return updatedChat, false
 }
 
+// refreshChatContext re-pins a chat to its agent's latest context snapshot
+// and clears the dirty marker.
+//
+// @Summary Refresh chat context
+// @ID refresh-chat-context
+// @Security CoderSessionToken
+// @Tags Chats
+// @Produce json
+// @Param chat path string true "Chat ID" format(uuid)
+// @Success 200 {object} codersdk.Chat
+// @Router /api/experimental/chats/{chat}/context [put]
+// @Description Experimental: this endpoint is subject to change.
+func (api *API) refreshChatContext(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	chat := httpmw.ChatParam(r)
+
+	if !api.Authorize(r, policy.ActionUpdate, chat.RBACObject()) {
+		httpapi.ResourceNotFound(rw)
+		return
+	}
+
+	updated, err := api.chatDaemon.RefreshChatContext(ctx, chat)
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error refreshing chat context.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+
+	httpapi.Write(ctx, rw, http.StatusOK, db2sdk.Chat(updated, nil, nil))
+}
+
 // patchChat updates a chat resource. Supports updating labels,
 // workspace binding, archiving, pinning, and pinned-chat ordering.
 //