Add JSON output to operational commands

[sauravpanda]

Summary

• add stable JSON schemas to stats, status, and sync
• keep stdout machine-readable by suppressing progress and capturing git output in JSON mode
• report per-worktree sync outcomes as synced, skipped, or failed
• document the agent- and CI-friendly command forms

Validation

• go test ./...
• go vet ./...
• go build ./...
• parsed real stats --json, status --json, and sync --dry-run --json output with jq

Closes #58