revert(chat): remove Sim Chat and mothership changes

Reverts this branch's Sim Chat surface and its mothership-view edits.

d4bdb87 ("feat(cli): add interactive Sim chat") could not be reverted: it
is a 175-file commit that also introduced the ExecutionContext refactor and
the v2 workspaces API, which 33 files under lib/copilot now depend on.
Reverting it produced 42 content conflicts, 24 of them in the refactor rather
than in chat code. Its chat contribution is removed forward instead.

Reverted:
  b25e7ba fix(copilot): recover tool arguments lost when checkpointed
  d4c7464 feat(cli): add resumable async Sim Chat
  3f0c1fc feat(cli): add saved chat commands
  de26320 feat(cli): refine chat and desktop updates -- mothership-view
             only; desktop updater, terminal themes, and bridge are kept

Removed forward: the 17 CLI chat modules, /api/v2/chat, /api/v2/chats, the v2
chat contracts, and lib/copilot/headless. lib/copilot/chat/turn-persistence.ts
is kept because the web chat's post.ts imports it.

Left alone: upstream Copilot application-boundary work (#6450-#6462), the
sim-chat billing source, and the v2 workspaces API.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JNFjchn6dcM7xevh34PKHE

Author: TheodoreSpeaks