Add status command, deployment infrastructure, and fix proto paths

Rename proto/gen directories from mc-proxy to mc_proxy for valid protobuf
package naming. Add CLI status subcommand for querying running instance
health via gRPC. Add systemd backup service/timer and backup pruning
script. Add buf.yaml and proto-lint Makefile target. Add shutdown_timeout
config field.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/skills/checkpoint/SKILL.md

@@ -0,0 +1,8 @@
+# Checkpoint Skill
+
+1. Run `go build ./...`  abort if errors
+2. Run `go test ./...`  abort if failures
+3. Run `go vet ./...`
+4. Run `git add -A && git status`  show user what will be committed
+5. Generate an appropriate commit message based on your instructions.
+6. Run `git commit -m "<message>"` and verify with `git log -1`