feat(run): explain roar register after the next-steps hint

[christophergeyer]

Summary

The post-`roar run` hint block already nudges:

```
hint: next: roar show --job abc12345 · roar dag · roar register @3
```

…but `roar register @3` is opaque to first-time users (and to coding agents reading the output programmatically). Adds a one-line explainer right after:

```
hint: next: roar show --job abc12345 · roar dag · roar register @3
'roar register @3' uploads lineage to glaas.ai so you can reproduce it later.
```

Details

• Uses the same `register_arg` as the action line, so `@N` form and UID-fallback form stay synchronized.
• Inherits the existing stream + gate: quiet mode, pipe mode, and `hints.enabled = false` all still suppress both lines as a unit.
• Same `warn_amber` styling — the two lines render as a coherent block.

Independence

Not stacked on #109 (the hints-to-stderr change). The new line goes through the same `_print` + `style` calls as the existing hint, so it inherits whatever gating + stream choice is in effect — works with the old TTY-suppressed semantics today, and will compose cleanly with #109 if both merge.

Test plan

• `test_next_steps_hint_explains_what_register_does` — explainer presence + `@N` form parity with the action line.
• `test_next_steps_hint_explainer_uses_uid_form_when_no_step_number` — fallback path parity.
• Existing `_silent_in_pipe_mode`, `_silent_in_quiet_mode`, `_silent_when_hints_disabled` still pass — the explainer inherits the gate.
• 938 unit tests passing, 1 pre-existing skipped.
• `ruff check .` + `ruff format --check .` + `mypy roar` all clean.

🤖 Generated with Claude Code