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/mcp/util.go

@@ -0,0 +1,30 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package mcp
+
+import (
+	"encoding/json"
+
+	internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
+)
+
+func assert(cond bool, msg string) {
+	if !cond {
+		panic(msg)
+	}
+}
+
+// remarshal marshals from to JSON, and then unmarshals into to, which must be
+// a pointer type.
+func remarshal(from, to any) error {
+	data, err := json.Marshal(from)
+	if err != nil {
+		return err
+	}
+	if err := internaljson.Unmarshal(data, to); err != nil {
+		return err
+	}
+	return nil
+}