feat(mcp): built-in playbooks get steward_url from Steward itself

#5015 "Built-in host-agent playbooks make an MCP caller supply steward_url"

- run_playbook, create_schedule and set_alert_action fill a steward-builtin
  playbook's steward_url when the caller leaves it out or empty, from
  Settings → General → Public base URL, else the scheme and host the MCP
  request arrived on — the same source the web actions use.
- run_playbook's reply names what it filled (filled_by_steward), and
  get_playbook shows that value as the variable's default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/mcp/server.py

@@ -279,11 +279,14 @@ def mount_mcp(app) -> StewardMCPServer:
 
         caller_token = _context._caller_ctx.set(caller)
         app_token = _context._app_ctx.set(app)
+        host = headers.get("host", "")
+        url_token = _context._base_url_ctx.set(f"{scope.get('scheme', 'http')}://{host}" if host else "")
         try:
             await mcp_asgi(scope, receive, send)
         finally:
             _context._caller_ctx.reset(caller_token)
             _context._app_ctx.reset(app_token)
+            _context._base_url_ctx.reset(url_token)
 
     original_asgi = app.asgi_app