feat: agents git watch backend (#22565)

Adds real-time git status watching for workspace agents, so the frontend
can subscribe over WebSocket and show
git file changes in near real-time.

1. Subscription is scoped to a **chat** via `GET
/api/experimental/chats/{chat}/git/watch`.
2. The workspace agent automatically determines which paths to watch
based on tool calls made by the chat (and its ancestor chats).
3. Workspace agent polls subscribed repo working trees on a 30s
interval, on tools calls, and on explicit `refresh` from the client.
4. Scans are rate-limited to at most once per second.
5. Edited paths are tracked **in-memory** inside the workspace agent.
There is no database persistence — state is lost on agent restart. This
will be addresses in a future PR.
6. Messages sent over WebSocket include a full-repo snapshot (unified
diff, branch, origin). A new message is emitted only when the snapshot
changes.

This PR was implemented with AI with me closely controlling what it's
doing. The code follows a plan file that was updated continuously during
implementation. Here's the file if you'd like to see it:
[project.md](https://gist.github.com/hugodutka/8722cf80c92f8a56555f7bc595b770e2).
It reflects the current state of the PR.

agent/agentgit/api.go

@@ -0,0 +1,141 @@
+package agentgit
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/google/uuid"
+
+	"cdr.dev/slog/v3"
+	"github.com/coder/coder/v2/coderd/httpapi"
+	"github.com/coder/coder/v2/codersdk"
+	"github.com/coder/coder/v2/codersdk/wsjson"
+	"github.com/coder/websocket"
+)
+
+// API exposes the git watch HTTP routes for the agent.
+type API struct {
+	logger    slog.Logger
+	opts      []Option
+	pathStore *PathStore
+}
+
+// NewAPI creates a new git watch API.
+func NewAPI(logger slog.Logger, pathStore *PathStore, opts ...Option) *API {
+	return &API{
+		logger:    logger,
+		pathStore: pathStore,
+		opts:      opts,
+	}
+}
+
+// Routes returns the chi router for mounting at /api/v0/git.
+func (a *API) Routes() http.Handler {
+	r := chi.NewRouter()
+	r.Get("/watch", a.handleWatch)
+	return r
+}
+
+func (a *API) handleWatch(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+
+	conn, err := websocket.Accept(rw, r, &websocket.AcceptOptions{
+		CompressionMode: websocket.CompressionNoContextTakeover,
+	})
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Failed to accept WebSocket.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+
+	// 4 MiB read limit — subscribe messages with many paths can exceed the
+	// default 32 KB limit. Matches the SDK/proxy side.
+	conn.SetReadLimit(1 << 22)
+
+	stream := wsjson.NewStream[
+		codersdk.WorkspaceAgentGitClientMessage,
+		codersdk.WorkspaceAgentGitServerMessage,
+	](conn, websocket.MessageText, websocket.MessageText, a.logger)
+
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	go httpapi.HeartbeatClose(ctx, a.logger, cancel, conn)
+
+	handler := NewHandler(a.logger, a.opts...)
+
+	// scanAndSend performs a scan and sends results if there are
+	// changes.
+	scanAndSend := func() {
+		msg := handler.Scan(ctx)
+		if msg != nil {
+			if err := stream.Send(*msg); err != nil {
+				a.logger.Debug(ctx, "failed to send changes", slog.Error(err))
+				cancel()
+			}
+		}
+	}
+
+	// If a chat_id query parameter is provided and the PathStore is
+	// available, subscribe to path updates for this chat.
+	chatIDStr := r.URL.Query().Get("chat_id")
+	if chatIDStr != "" && a.pathStore != nil {
+		chatID, parseErr := uuid.Parse(chatIDStr)
+		if parseErr == nil {
+			// Load any paths that are already tracked for this chat.
+			existingPaths := a.pathStore.GetPaths(chatID)
+			if len(existingPaths) > 0 {
+				handler.Subscribe(existingPaths)
+				handler.RequestScan()
+			}
+			// Subscribe to future path updates.
+			notifyCh, unsubscribe := a.pathStore.Subscribe(chatID)
+			defer unsubscribe()
+
+			go func() {
+				for {
+					select {
+					case <-ctx.Done():
+						return
+					case <-notifyCh:
+						paths := a.pathStore.GetPaths(chatID)
+						handler.Subscribe(paths)
+						handler.RequestScan()
+					}
+				}
+			}()
+		}
+	}
+
+	// Start the main run loop in a goroutine.
+	go handler.RunLoop(ctx, scanAndSend)
+
+	// Read client messages.
+	updates := stream.Chan()
+	for {
+		select {
+		case <-ctx.Done():
+			_ = stream.Close(websocket.StatusGoingAway)
+			return
+		case msg, ok := <-updates:
+			if !ok {
+				return
+			}
+
+			switch msg.Type {
+			case codersdk.WorkspaceAgentGitClientMessageTypeRefresh:
+				handler.RequestScan()
+			default:
+				if err := stream.Send(codersdk.WorkspaceAgentGitServerMessage{
+					Type:    codersdk.WorkspaceAgentGitServerMessageTypeError,
+					Message: "unknown message type",
+				}); err != nil {
+					return
+				}
+			}
+		}
+	}
+}