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>

tests/mcp/test_automation.py

@@ -107,3 +107,28 @@ def test_a_viewer_token_cannot_start_a_run(app):
     assert "needs the operator role" in app.mcp_server.refusal("run_playbook", viewer)
     assert app.mcp_server.refusal("run_playbook", operator) is None
     assert "needs the admin role" in app.mcp_server.refusal("set_alert_action", operator)
+
+
+def test_builtin_playbooks_get_steward_url_from_steward():
+    from steward.mcp import _context
+    app = NS(config={"PUBLIC_BASE_URL": ""})
+    a, u = _context._app_ctx.set(app), _context._base_url_ctx.set("http://steward.lan")
+    try:
+        declared = [{"name": "steward_url", "default": "", "secret": False, "required": False,
+                     "prompt": None}]
+        builtin = {"name": "steward-builtin"}
+        got, filled = automation.steward_defaults(builtin, declared, {"agent_interval": 30})
+        assert got == {"agent_interval": 30, "steward_url": "http://steward.lan"}
+        assert filled == {"steward_url": "http://steward.lan"}
+        # The configured public URL wins over the address the request came in on.
+        app.config["PUBLIC_BASE_URL"] = "https://steward.example/"
+        assert automation.steward_defaults(builtin, declared, {})[1] == {
+            "steward_url": "https://steward.example"}
+        # A value the caller passed stands; another source's playbook is untouched.
+        assert automation.steward_defaults(builtin, declared, {"steward_url": "http://x"})[1] == {}
+        assert automation.steward_defaults({"name": "homelab"}, declared, {}) == ({}, {})
+        # A playbook that doesn't declare it gets nothing extra.
+        assert automation.steward_defaults(builtin, [], {}) == ({}, {})
+    finally:
+        _context._app_ctx.reset(a)
+        _context._base_url_ctx.reset(u)