[chrisnestrud]

Honor reduced motion in the Textual TUI

Summary

Makes the TUI respect existing motion controls instead of adding a new Cecli
option: --no-spinner now also gates the footer spinner, and Textual's native
TEXTUAL_ANIMATIONS=none disables frame animation and icon alternation while
preserving status and polling.

Changes

• cecli/tui/__init__.py: pass show_spinner=args.spinner to the adapter.
• cecli/tui/io.py: gate spinner start/update/stop on spinner_active.
• cecli/tui/widgets/footer.py: skip the animation interval and render a
  static indicator when animation_level == "none".
• cecli/tui/widgets/input_container.py: stop alternating sub-agent icons
  under reduced motion while keeping polling.
• cecli/website/docs/config/tui.md: "Reduce Motion" section.
• Tests for spinner suppression and reduced-motion rendering.

Testing

New TUI and spinner tests pass; no new config option or dependency.