docs: revise worktree design to be TUI-first

Remove CLI subcommands (chief worktree new/list/switch) in favor of
TUI-native flows:

- New PRD dialog includes worktree creation toggle
- Dashboard IS the worktree list (no separate command)
- [A] hotkey for coordinator mode showing all worktrees
- [E] hotkey for inline environment control
- Tab for quick worktree switching via enhanced picker
- Terminal spawning when selecting different worktree

This matches Chief's existing design philosophy where the TUI is the
primary interface, not CLI subcommands.

Updated user journeys to show actual TUI flows with ASCII mockups.

https://claude.ai/code/session_01C5LeRjf7VdBAoZNywrTwMN

Author: claude