feat(agent): session-ready webhook for SSE-driven player handshake (0.9.13)

Closes Fase 3.3b. Daemon now tells the server the moment a session's
first HLS segment + init.mp4 land on disk; the web side flips
streaming_session.ready_at = NOW(), which its SSE endpoint pushes to
subscribed players so the loading UI flips from "Preparando…" to
"Stream listo" without polling HEAD on the segment URL.

Surface:
  - New Client.MarkSessionReady(ctx, sessionId) HTTP method →
    POST /api/internal/agent/session-ready.
  - New engine.HLSSession.ReadyCount() + FromCache() accessors so the
    watcher goroutine doesn't reach into private state.
  - New cmd.watchSessionReady(ctx, client, hsess, sessionId) goroutine
    polls ReadyCount every 200 ms with a 60 s deadline + short-circuits
    for cache-HIT sessions (ready the moment StartHLSSession returns).
  - Daemon callback spawns it right after streamSrv.HLS().Register so
    the watcher's lifecycle matches the session's.

Best-effort: a transient network failure on the webhook is logged + the
goroutine exits — the player's existing HEAD-probe retry path still
discovers ready state independently. The webhook is an acceleration,
not a hard dependency.

internal/agent/client.go

@@ -109,6 +109,27 @@ func (c *Client) ReportUpgradeResult(ctx context.Context, agentID string, succes
 	return nil
 }
 
+// MarkSessionReady signals the server that the first HLS segment + init.mp4
+// landed on disk for the given session. The web side flips
+// streaming_session.ready_at = NOW(), which its SSE endpoint emits to
+// subscribed players so the "Preparando…" UI ends without polling HEAD
+// on /hls/<id>/master.m3u8.
+//
+// Best-effort: the server is the source of truth for session state and
+// will reach the same conclusion via HEAD probes anyway if this call
+// fails. We log the error in the caller but don't retry — by the time
+// a retry would land the user is likely already playing.
+func (c *Client) MarkSessionReady(ctx context.Context, sessionID string) error {
+	req := struct {
+		SessionID string `json:"sessionId"`
+	}{SessionID: sessionID}
+	var resp StatusResponse
+	if err := c.doPost(ctx, "/api/internal/agent/session-ready", req, &resp); err != nil {
+		return fmt.Errorf("mark session ready: %w", err)
+	}
+	return nil
+}
+
 // ReportStatus reports download progress. Returns server-side flags the CLI must act on.
 func (c *Client) ReportStatus(ctx context.Context, update StatusUpdate) (*StatusResponse, error) {
 	var resp StatusResponse