fix(docs): align API-001 --format values with implementation [hx-34bff005]

Remove stale 'prompt' format value and update default from 'prompt' to
'json' in API-001-dun-cli.md line 25, matching cmd/dun/main.go after
commit 0e8e8b4 (hx-4e05d540).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: easel

docs/design/contracts/API-001-dun-cli.md

@@ -22,7 +22,7 @@ $ dun [command] [options] [arguments]
 **Usage**: `$ dun check [options]`
 
 **Options**:
-- `--format` : Output format (`prompt`, `llm`, or `json`, default `prompt`)
+- `--format` : Output format (`llm` or `json`, default `json`)
 - `--changed` : Limit checks to changed files (default `false`)
 - `--timeout` : Global timeout in seconds (default `600`)
 - `--check-timeout` : Per-check timeout in seconds (default `120`)