feat(plugin): add /api/plugin/context push-channel endpoint + dogfood hook

Phase 1 of plan #755 (Scribe-as-plugin). Gives Scribe its own session-start
push channel so always-on rules + active-project context surface without being
asked — the gap behind 'I have to prompt for everything'.

- services/plugin_context.build_session_context: renders always-on rule titles
  grouped by topic (under the 10k additionalContext cap; full text stays one
  list_always_on_rules/get_rule call away) + optional project goal/open-task
  count + a recall/update-over-create reflex line. Capped at 9000 chars.
- routes/plugin GET /api/plugin/context (login_required already accepts Bearer
  fmcp_ keys; read scope suffices).
- tests: titles-not-statements, project scoping, length cap (pure mocks).
- scripts/scribe_session_context.sh: dogfood SessionStart hook, fail-open,
  reads url+token from .mcp.json. Superseded in Phase 2 by the plugin-bundled
  hook using userConfig.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/scribe/app.py

@@ -26,6 +26,7 @@ from scribe.routes.search import search_bp
 from scribe.routes.profile import profile_bp
 from scribe.routes.knowledge import knowledge_bp
 from scribe.routes.rulebooks import rulebooks_bp
+from scribe.routes.plugin import plugin_bp
 from scribe.routes.trash import trash_bp
 from scribe.routes.dashboard import dashboard_bp
 from scribe.mcp import mount_mcp
@@ -87,6 +88,7 @@ def create_app() -> Quart:
     app.register_blueprint(profile_bp)
     app.register_blueprint(knowledge_bp)
     app.register_blueprint(rulebooks_bp)
+    app.register_blueprint(plugin_bp)
     app.register_blueprint(trash_bp)
     app.register_blueprint(dashboard_bp)