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/session.go

@@ -0,0 +1,29 @@
+// 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
+
+// hasSessionID is the interface which, if implemented by connections, informs
+// the session about their session ID.
+//
+// TODO(rfindley): remove SessionID methods from connections, when it doesn't
+// make sense. Or remove it from the Sessions entirely: why does it even need
+// to be exposed?
+type hasSessionID interface {
+	SessionID() string
+}
+
+// ServerSessionState is the state of a session.
+type ServerSessionState struct {
+	// InitializeParams are the parameters from 'initialize'.
+	InitializeParams *InitializeParams `json:"initializeParams"`
+
+	// InitializedParams are the parameters from 'notifications/initialized'.
+	InitializedParams *InitializedParams `json:"initializedParams"`
+
+	// LogLevel is the logging level for the session.
+	LogLevel LoggingLevel `json:"logLevel"`
+
+	// TODO: resource subscriptions
+}