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.

CHANGELOG.md

@@ -5,6 +5,21 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.9.13] - 2026-05-27
+
+### Added
+
+- **Session-ready webhook** (`/api/internal/agent/session-ready`). Daemon
+  watches every new HLSSession's segment counter and, the moment seg-0 +
+  init.mp4 land on disk, POSTs the sessionId to the server. The web side
+  flips `streaming_session.ready_at = NOW()`, which its new SSE endpoint
+  pushes to subscribed players so the "Preparando…" UI flips to
+  "Stream listo" without waiting for the player's HEAD-probe retry loop
+  to discover it. Cache-HIT sessions fire the webhook immediately on
+  StartHLSSession return.
+- `engine.HLSSession.ReadyCount()` + `FromCache()` accessors so the
+  ready-watcher goroutine doesn't reach into private state.
+
 ## [0.9.12] - 2026-05-27
 
 ### Added