refactor: cli as a channel

[PsiACE]

This PR moves interactive CLI I/O into the channel system
and keeps behavior aligned with existing message channels.

What changed

• Added a new CliChannel and wired bub chat through
  ChannelManager (instead of hardcoded CLI loop).
• Kept InteractiveCli as a thin compatibility wrapper over
  CliChannel.
• Extended channel abstractions:
  • BaseChannel.debounce_enabled
  • BaseChannel.format_prompt
• Updated SessionRunner:
  • Immediate execution for non-debounced channels (CLI)
  • Prevented command prompt leakage into batched
    prompts
  • CLI path is now fail-fast on runtime errors
• Added duplicate channel registration guard in
  ChannelManager.
• Updated docs:
  • CLI described as a local channel adapter
  • Channels overview updated
  • Nav groups CLI under Channels

Validation

• Added/updated tests for chat channel wiring, session
  runner behavior, and duplicate channel registration.
• ruff, mypy, and full pytest pass.

[frostming]

Merge conflicts