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/util/net.go

@@ -0,0 +1,26 @@
+// Copyright 2025 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 util
+
+import (
+	"net"
+	"net/netip"
+	"strings"
+)
+
+func IsLoopback(addr string) bool {
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		// If SplitHostPort fails, it might be just a host without a port.
+		host = strings.Trim(addr, "[]")
+	}
+	if host == "localhost" {
+		return true
+	}
+	ip, err := netip.ParseAddr(host)
+	if err != nil {
+		return false
+	}
+	return ip.IsLoopback()
+}