[Request] Plain line output for spinner and progress in CI #629

Is your feature request related to a problem? Please describe.

When CI is "true", spinner() and progress() stop the animation. But the CI log still gets
terminal output that only makes sense on a TTY. This is the output of changeset publish
(changesets uses progress()) on GitHub Actions, with CI=true:

^[[?25l◒  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Publishing packages...
^[[1G^[[J◇  Successfully published:
...
^[[?25h^[[?25l◒  Creating git tags...
^[[1G^[[J◇  Created git tags.
^[[?25h

Progress updates also print lines like this one, with a new frame each time and no message:

◐  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

The problems:

1. Cursor codes: ESC[?25l and ESC[?25h from block(), and ESC[1G with ESC[J from the
   clear step. A CI log viewer shows them as noise.
2. The spinner frame (◒, ◐) on each line. A frame of an animation means nothing in a log.
3. The progress bar. Each update prints one more bar line, often with no message.
4. Each change of the spinner message prints a new line. That is fine for real steps, but a
   message that only updates a counter or a string fills the log.

Describe the solution you'd like

In CI mode (or when the output is not a TTY), spinner(), progress() and the other live
components write plain lines only:

• No cursor or erase codes, and no call to block() that hides the cursor.
• No spinner frame and no progress bar. Print the message, then the final status line
  (◇ Done, ■ Cancelled, ▲ Failed).
• Optional: an option to skip message updates in CI, so only the start and the end lines print.

Describe alternatives you've considered

• Strip ANSI codes in each consumer, for example with a pipe through sed. Every CLI that uses
  clack must then do it, and a pipe can hide the exit code.
• Consider using isTTY for isCI #286 (use isTTY for isCI). That changes when CI mode turns on. This request is about what
  CI mode prints. Both changes work well together.

Additional context

• @clack/prompts 1.8.1, @clack/core 1.5.1, through @changesets/cli 3.0.3.
• Node 26, GitHub Actions runner.