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/_context.py

@@ -18,6 +18,8 @@ if TYPE_CHECKING:
 
 _caller_ctx: ContextVar["Caller | None"] = ContextVar("steward_mcp_caller", default=None)
 _app_ctx: ContextVar["Quart | None"] = ContextVar("steward_mcp_app", default=None)
+# The scheme and host the caller reached /mcp on, e.g. "https://steward.example".
+_base_url_ctx: ContextVar[str] = ContextVar("steward_mcp_base_url", default="")
 
 
 def current_caller() -> "Caller":
@@ -36,3 +38,13 @@ def mcp_app() -> "Quart":
     if app is None:
         raise RuntimeError("no Steward app in MCP context")
     return app
+
+
+def public_url() -> str:
+    """Steward's externally reachable base URL, as the web side's
+    public_base_url() gives it: Settings → General → Public base URL when set,
+    else the scheme and host this MCP request arrived on; "" when neither is
+    known."""
+    app = _app_ctx.get()
+    configured = (app.config.get("PUBLIC_BASE_URL") or "").strip() if app is not None else ""
+    return (configured or _base_url_ctx.get()).rstrip("/")