chore(cleanup): re-delete rich-progress (dead code; not a v6-migration accident)

The earlier deletion at `aaeb57e04` was correct: rich-progress.mts has
zero non-test importers anywhere in the codebase. The intervening
`e01a6c814 fix(rich-progress): restore inadvertently-deleted file`
misdiagnosed the deletion as a v6-migration accident and brought it
back.

Confirmed dead:
  - grep across packages/cli/src, packages/cli/scripts, packages/cli/bin,
    packages/build-infra, packages/package-builder finds zero non-test
    importers of `rich-progress` / `MultiProgress` / `Spinner` /
    `FileProgress`.
  - The original 2025 commit `a89d5a553` introduced the file as a
    'foundation for Claude CLI-like enhancements' — speculative; never
    wired up.

Keep deleted.

Author: jdalton