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/apidoc/docs.go

@@ -549,6 +549,42 @@ const docTemplate = `{
                 }
             }
         },
+        "/api/experimental/chats/{chat}/context": {
+            "put": {
+                "description": "Experimental: this endpoint is subject to change.",
+                "produces": [
+                    "application/json"
+                ],
+                "tags": [
+                    "Chats"
+                ],
+                "summary": "Refresh chat context",
+                "operationId": "refresh-chat-context",
+                "parameters": [
+                    {
+                        "type": "string",
+                        "format": "uuid",
+                        "description": "Chat ID",
+                        "name": "chat",
+                        "in": "path",
+                        "required": true
+                    }
+                ],
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/codersdk.Chat"
+                        }
+                    }
+                },
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ]
+            }
+        },
         "/api/experimental/chats/{chat}/diff": {
             "get": {
                 "description": "Experimental: this endpoint is subject to change.",
@@ -16450,6 +16486,14 @@ const docTemplate = `{
                 "client_type": {
                     "$ref": "#/definitions/codersdk.ChatClientType"
                 },
+                "context": {
+                    "description": "Context reports the chat's pinned workspace-context state and\nwhether it has drifted from the agent's latest pushed snapshot.\nNil when the chat has no pinned context yet.",
+                    "allOf": [
+                        {
+                            "$ref": "#/definitions/codersdk.ChatContext"
+                        }
+                    ]
+                },
                 "created_at": {
                     "type": "string",
                     "format": "date-time"
@@ -16605,6 +16649,24 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.ChatContext": {
+            "type": "object",
+            "properties": {
+                "dirty": {
+                    "description": "Dirty is true when the agent's latest snapshot hash differs from the\nchat's pinned hash.",
+                    "type": "boolean"
+                },
+                "dirty_since": {
+                    "description": "DirtySince is when drift was first detected; nil when not dirty.",
+                    "type": "string",
+                    "format": "date-time"
+                },
+                "error": {
+                    "description": "Error is the snapshot-level error copied from the pinned snapshot\n(empty when healthy).",
+                    "type": "string"
+                }
+            }
+        },
         "codersdk.ChatDiffContents": {
             "type": "object",
             "properties": {
@@ -17544,7 +17606,8 @@ const docTemplate = `{
                 "created",
                 "deleted",
                 "diff_status_change",
-                "action_required"
+                "action_required",
+                "context_dirty"
             ],
             "x-enum-varnames": [
                 "ChatWatchEventKindStatusChange",
@@ -17553,7 +17616,8 @@ const docTemplate = `{
                 "ChatWatchEventKindCreated",
                 "ChatWatchEventKindDeleted",
                 "ChatWatchEventKindDiffStatusChange",
-                "ChatWatchEventKindActionRequired"
+                "ChatWatchEventKindActionRequired",
+                "ChatWatchEventKindContextDirty"
             ]
         },
         "codersdk.ConnectionLatency": {