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/yosida95/uritemplate/v3/error.go

@@ -0,0 +1,16 @@
+// Copyright (C) 2016 Kohei YOSHIDA. All rights reserved.
+//
+// This program is free software; you can redistribute it and/or
+// modify it under the terms of The BSD 3-Clause License
+// that can be found in the LICENSE file.
+
+package uritemplate
+
+import (
+	"fmt"
+)
+
+func errorf(pos int, format string, a ...interface{}) error {
+	msg := fmt.Sprintf(format, a...)
+	return fmt.Errorf("uritemplate:%d:%s", pos, msg)
+}