feat: add MCP endpoint exposing calendar tools over Streamable HTTP

Adds a POST /mcp endpoint (github.com/modelcontextprotocol/go-sdk) with
three tools mirroring the existing /calendar logic: get_calendar_today,
get_calendar_for_date and get_holidays. Bumps go.mod to Go 1.25 (required
by the SDK) and vendors the new dependencies. Also fixes an unanchored
.gitignore rule ("domogeek") that was silently excluding all untracked
files under cmd/domogeek/.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Hu8wwahgecuCGfesTKCfEd

vendor/github.com/modelcontextprotocol/go-sdk/internal/authutil/util.go

@@ -0,0 +1,13 @@
+// Copyright 2026 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+package authutil
+
+import "strings"
+
+// IssuersEqual reports whether two OAuth 2.0 authorization server issuer
+// identifiers refer to the same server comparing them without the final trailing slash.
+func IssuersEqual(a, b string) bool {
+	return strings.TrimSuffix(a, "/") == strings.TrimSuffix(b, "/")
+}