zen CLI: suggest similar commands on typos (#1000)

Surface "did you mean?" suggestions when the `zen` CLI is invoked with an unknown command or subcommand, so users don't have to dig through `zen --help` every time they mistype.

```
$ zen stauts
Unknown command specified: 'stauts'

The most similar commands are:
  status

Run 'zen --help' for the full list of commands.
```

```
$ zen cache statz
Unknown subcommand: 'statz'

The most similar subcommands are:
  stats
```

## Algorithm

- Damerau-Levenshtein edit distance with case-insensitive ASCII comparison — handles insertions, deletions, substitutions, and adjacent transpositions (e.g. `versoin` → `version`).
- Small prefix-match bonus so short inputs like `ca` still surface longer commands like `cache` without having to relax the distance threshold to the point where it admits noise.
- Distance threshold scales with input length (`clamp(len/2, 1, 3)`). Very short inputs rely on the prefix bonus; longer inputs tolerate up to three edits.
- Top 5 results by distance, stable-sorted.
- Hidden commands (deprecated shims like `cache-stats`) are excluded from the candidate set so we don't advertise them.

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