fix(container): surface docker health status in rtk docker ps output

[Donotavio]

Problem

`rtk docker ps` fetches `{{.Status}}` which already includes health state when configured:
```
Up 2 hours (healthy)
Up 1 hour (unhealthy)
```

But `docker_ps()` parsed `parts[2]` (Status field) and discarded it completely. An AI agent monitoring containers had no way to identify unhealthy services.

Fix

• Include Status field in output between name and image
• Prefix unhealthy containers with `[!]` for immediate visibility
• All other states (healthy, plain Up, Exited, Restarting) pass through verbatim

Refactoring

• Extract `format_docker_ps_line(parts) -> Option` as `pub(crate)` pure function — directly unit-testable without spawning docker
• Extract `trim_image_registry()` helper — eliminates duplication with `format_compose_ps()` which had the identical `.split('/').next_back()` pattern
• Add `UNHEALTHY_PREFIX` constant

Tests

7 new unit tests:

• `test_docker_ps_preserves_healthy_status` — (healthy) shown verbatim, no [!]
• `test_docker_ps_marks_unhealthy_prominently` — [!] prefix present
• `test_docker_ps_shows_status_without_health_check` — plain Up shown
• `test_docker_ps_exited_no_flag` — Exited shown, no [!]
• `test_docker_ps_restarting_no_flag` — Restarting shown, no [!]
• `test_docker_ps_line_too_few_parts_returns_none` — None guard for malformed input
• `test_docker_ps_formats_all_five_containers` — all 5 states in one pass

---

Identified via adversarial testing: https://www.reddit.com/r/ClaudeCode/comments/1spiy8t/

[CLAassistant]

All committers have signed the CLA.