Add experimental label and improve descriptions

[matthewborden]

Description

./bk --help
....
  preflight run [flags]
    Run a build against a snapshot of the local working tree (experimental)

  preflight cleanup [flags]
    Clean up completed preflight branches (experimental)

Usage: bk preflight <command> [flags]

Run a build against a snapshot of the local working tree (experimental)

Preflight is an experimental preview and subject to change without notice.

Snapshots your working tree (staged, unstaged, and untracked files) to a temporary commit on a bk/preflight/<id> branch, triggers a build on the selected pipeline,
monitors failures, exits as soon as the build starts failing, and cleans up the temporary branch when finished.

Flags:
  -h, --help        Show context-sensitive help.
  -y, --yes         Skip all confirmation prompts
      --no-input    Disable all interactive prompts
  -q, --quiet       Suppress progress output
      --no-pager    Disable pager for text output
      --debug       Enable debug output for REST API calls

Commands:
  preflight run [flags]
    Run a build against a snapshot of the local working tree (experimental)

  preflight cleanup [flags]
    Clean up completed preflight branches (experimental)

matthewborden@Matthews-MacBook-Pro-2:~/cli  % ./bk preflight run --help
Usage: bk preflight run [flags]

Run a build against a snapshot of the local working tree (experimental)

Preflight is an experimental preview and subject to change without notice.

Snapshots your working tree (staged, unstaged, and untracked files) to a temporary commit on a bk/preflight/<id> branch, triggers a build on the selected pipeline,
monitors failures, exits as soon as the build starts failing, and cleans up the temporary branch when finished.

Flags:
  -h, --help                   Show context-sensitive help.
  -y, --yes                    Skip all confirmation prompts
      --no-input               Disable all interactive prompts
  -q, --quiet                  Suppress progress output
      --no-pager               Disable pager for text output
      --debug                  Enable debug output for REST API calls

  -p, --pipeline=STRING        The pipeline to build. This can be a {pipeline slug} or in the format {org slug}/{pipeline slug}.
      --[no-]watch             Watch the build until completion.
      --exit-on=EXIT-ON,...    Exit when a condition is met. Options: build-failing (default, exits when a build enters the failing state), build-terminal (exits when
                               the build reaches a terminal state).
      --interval=2             Polling interval in seconds when watching.
      --no-cleanup             Skip cleanup after completion or early exit. The preflight branch remains and the build keeps running if exiting early.
      --await-test-results     After the build finishes, wait for test results to be processed by Test Engine. Provide a duration like 10s, or omit the value to wait
                               30s.
      --text                   Use plain text output instead of interactive terminal UI.
      --json                   Emit one JSON object per event (JSONL).