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/cmd/version.go

@@ -1,4 +1,4 @@
 package cmd
 
 // Version is the CLI version. Overridden by goreleaser ldflags at release time.
-var Version = "0.9.12"
+var Version = "0.9.13"