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/jsonrpc2/jsonrpc2.go

@@ -0,0 +1,63 @@
+// Copyright 2018 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 jsonrpc2 is a minimal implementation of the JSON RPC 2 spec.
+// https://www.jsonrpc.org/specification
+// It is intended to be compatible with other implementations at the wire level.
+package jsonrpc2
+
+import (
+	"context"
+	"errors"
+)
+
+// ErrNotHandled is returned from a Handler or Preempter to indicate it did
+// not handle the request.
+//
+// If a Handler returns ErrNotHandled, the server replies with
+// ErrMethodNotFound.
+var ErrNotHandled = errors.New("JSON RPC not handled")
+
+// Preempter handles messages on a connection before they are queued to the main
+// handler.
+// Primarily this is used for cancel handlers or notifications for which out of
+// order processing is not an issue.
+type Preempter interface {
+	// Preempt is invoked for each incoming request before it is queued for handling.
+	//
+	// If Preempt returns ErrNotHandled, the request will be queued,
+	// and eventually passed to a Handle call.
+	//
+	// Otherwise, the result and error are processed as if returned by Handle.
+	//
+	// Preempt must not block. (The Context passed to it is for Values only.)
+	Preempt(ctx context.Context, req *Request) (result any, err error)
+}
+
+// Handler handles messages on a connection.
+type Handler interface {
+	// Handle is invoked sequentially for each incoming request that has not
+	// already been handled by a Preempter.
+	//
+	// If the Request has a nil ID, Handle must return a nil result,
+	// and any error may be logged but will not be reported to the caller.
+	//
+	// If the Request has a non-nil ID, Handle must return either a
+	// non-nil, JSON-marshalable result, or a non-nil error.
+	//
+	// The Context passed to Handle will be canceled if the
+	// connection is broken or the request is canceled or completed.
+	// (If Handle returns ErrAsyncResponse, ctx will remain uncanceled
+	// until either Cancel or Respond is called for the request's ID.)
+	Handle(ctx context.Context, req *Request) (result any, err error)
+}
+
+// A HandlerFunc implements the Handler interface for a standalone Handle function.
+type HandlerFunc func(ctx context.Context, req *Request) (any, error)
+
+func (f HandlerFunc) Handle(ctx context.Context, req *Request) (any, error) {
+	return f(ctx, req)
+}
+
+var _ Handler = HandlerFunc(nil)