feat: add --json flag to `rafter agent status`

[Rome-1]

What

rafter agent status only prints formatted text. Add a --json flag for programmatic/agent consumption.

Why

Other commands like rafter agent audit --json and rafter agent scan --json already support structured output. AI agents and CI scripts need machine-readable status checks.

Files to change

1. node/src/commands/agent/status.ts — add --json option, output JSON to stdout
2. python/rafter_cli/commands/agent.py — add matching --json flag to the status subcommand
3. Add tests in both node/tests/ and python/tests/
4. Update shared-docs/CLI_SPEC.md with the JSON schema

Expected JSON output

{
  "installed": true,
  "version": "0.6.5",
  "agents_detected": ["claude-code", "cursor"],
  "hooks_installed": ["pre-commit"],
  "gitleaks_available": true,
  "config_path": "~/.rafter/config.json",
  "audit_log_path": "~/.rafter/audit.jsonl"
}

Reference

See how rafter agent audit implements --json in node/src/commands/agent/audit.ts for the pattern.