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/base64/encode_amd64.go

@@ -0,0 +1,8 @@
+// Code generated by command: go run encode_asm.go -pkg base64 -out ../base64/encode_amd64.s -stubs ../base64/encode_amd64.go. DO NOT EDIT.
+
+//go:build !purego
+// +build !purego
+
+package base64
+
+func encodeAVX2(dst []byte, src []byte, lut *int8) (int, int)