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.go

@@ -0,0 +1,18 @@
+package ascii
+
+import "github.com/segmentio/asm/internal/unsafebytes"
+
+// Valid returns true if b contains only ASCII characters.
+func Valid(b []byte) bool {
+	return ValidString(unsafebytes.String(b))
+}
+
+// ValidBytes returns true if b is an ASCII character.
+func ValidByte(b byte) bool {
+	return b <= 0x7f
+}
+
+// ValidBytes returns true if b is an ASCII character.
+func ValidRune(r rune) bool {
+	return r <= 0x7f
+}