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/pathstore.go

@@ -0,0 +1,136 @@
+package agentgit
+
+import (
+	"sort"
+	"sync"
+
+	"github.com/google/uuid"
+)
+
+// PathStore tracks which file paths each chat has touched.
+// It is safe for concurrent use.
+type PathStore struct {
+	mu          sync.RWMutex
+	chatPaths   map[uuid.UUID]map[string]struct{}
+	subscribers map[uuid.UUID][]chan<- struct{}
+}
+
+// NewPathStore creates a new PathStore.
+func NewPathStore() *PathStore {
+	return &PathStore{
+		chatPaths:   make(map[uuid.UUID]map[string]struct{}),
+		subscribers: make(map[uuid.UUID][]chan<- struct{}),
+	}
+}
+
+// AddPaths adds paths to every chat in chatIDs and notifies
+// their subscribers. Zero-value UUIDs are silently skipped.
+func (ps *PathStore) AddPaths(chatIDs []uuid.UUID, paths []string) {
+	affected := make([]uuid.UUID, 0, len(chatIDs))
+	for _, id := range chatIDs {
+		if id != uuid.Nil {
+			affected = append(affected, id)
+		}
+	}
+	if len(affected) == 0 {
+		return
+	}
+
+	ps.mu.Lock()
+	for _, id := range affected {
+		m, ok := ps.chatPaths[id]
+		if !ok {
+			m = make(map[string]struct{})
+			ps.chatPaths[id] = m
+		}
+		for _, p := range paths {
+			m[p] = struct{}{}
+		}
+	}
+	ps.mu.Unlock()
+
+	ps.notifySubscribers(affected)
+}
+
+// Notify sends a signal to all subscribers of the given chat IDs
+// without adding any paths. Zero-value UUIDs are silently skipped.
+func (ps *PathStore) Notify(chatIDs []uuid.UUID) {
+	affected := make([]uuid.UUID, 0, len(chatIDs))
+	for _, id := range chatIDs {
+		if id != uuid.Nil {
+			affected = append(affected, id)
+		}
+	}
+	if len(affected) == 0 {
+		return
+	}
+	ps.notifySubscribers(affected)
+}
+
+// notifySubscribers sends a non-blocking signal to all subscriber
+// channels for the given chat IDs.
+func (ps *PathStore) notifySubscribers(chatIDs []uuid.UUID) {
+	ps.mu.RLock()
+	toNotify := make([]chan<- struct{}, 0)
+	for _, id := range chatIDs {
+		toNotify = append(toNotify, ps.subscribers[id]...)
+	}
+	ps.mu.RUnlock()
+
+	for _, ch := range toNotify {
+		select {
+		case ch <- struct{}{}:
+		default:
+		}
+	}
+}
+
+// GetPaths returns all paths tracked for a chat, deduplicated
+// and sorted lexicographically.
+func (ps *PathStore) GetPaths(chatID uuid.UUID) []string {
+	ps.mu.RLock()
+	defer ps.mu.RUnlock()
+
+	m := ps.chatPaths[chatID]
+	if len(m) == 0 {
+		return nil
+	}
+	out := make([]string, 0, len(m))
+	for p := range m {
+		out = append(out, p)
+	}
+	sort.Strings(out)
+	return out
+}
+
+// Len returns the number of chat IDs that have tracked paths.
+func (ps *PathStore) Len() int {
+	ps.mu.RLock()
+	defer ps.mu.RUnlock()
+	return len(ps.chatPaths)
+}
+
+// Subscribe returns a channel that receives a signal whenever
+// paths change for chatID, along with an unsubscribe function
+// that removes the channel.
+func (ps *PathStore) Subscribe(chatID uuid.UUID) (<-chan struct{}, func()) {
+	ch := make(chan struct{}, 1)
+
+	ps.mu.Lock()
+	ps.subscribers[chatID] = append(ps.subscribers[chatID], ch)
+	ps.mu.Unlock()
+
+	unsub := func() {
+		ps.mu.Lock()
+		defer ps.mu.Unlock()
+		subs := ps.subscribers[chatID]
+		for i, s := range subs {
+			if s == ch {
+				ps.subscribers[chatID] = append(subs[:i], subs[i+1:]...)
+				break
+			}
+		}
+	}
+
+	return ch, unsub
+}