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/asm/ascii/valid_default.go

@@ -0,0 +1,48 @@
+//go:build purego || !amd64
+// +build purego !amd64
+
+package ascii
+
+import (
+	"unsafe"
+)
+
+// ValidString returns true if s contains only ASCII characters.
+func ValidString(s string) bool {
+	p := *(*unsafe.Pointer)(unsafe.Pointer(&s))
+	i := uintptr(0)
+	n := uintptr(len(s))
+
+	for i+8 <= n {
+		if (*(*uint64)(unsafe.Pointer(uintptr(p) + i)) & 0x8080808080808080) != 0 {
+			return false
+		}
+		i += 8
+	}
+
+	if i+4 <= n {
+		if (*(*uint32)(unsafe.Pointer(uintptr(p) + i)) & 0x80808080) != 0 {
+			return false
+		}
+		i += 4
+	}
+
+	if i == n {
+		return true
+	}
+
+	p = unsafe.Pointer(uintptr(p) + i)
+
+	var x uint32
+	switch n - i {
+	case 3:
+		x = uint32(*(*uint16)(p)) | uint32(*(*uint8)(unsafe.Pointer(uintptr(p) + 2)))<<16
+	case 2:
+		x = uint32(*(*uint16)(p))
+	case 1:
+		x = uint32(*(*uint8)(p))
+	default:
+		return true
+	}
+	return (x & 0x80808080) == 0
+}