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/segmentio/encoding/json/reflect.go

@@ -0,0 +1,20 @@
+//go:build go1.20
+// +build go1.20
+
+package json
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+func extendSlice(t reflect.Type, s *slice, n int) slice {
+	arrayType := reflect.ArrayOf(n, t.Elem())
+	arrayData := reflect.New(arrayType)
+	reflect.Copy(arrayData.Elem(), reflect.NewAt(t, unsafe.Pointer(s)).Elem())
+	return slice{
+		data: unsafe.Pointer(arrayData.Pointer()),
+		len:  s.len,
+		cap:  n,
+	}
+}