fix(commands): implement all slash command handlers in runCommand

Previously only /tasks, /verification, /budget, /sessions, /help,
/proxy, /clear, /mode, and /login were wired up in the runCommand
switch. All other commands (/auth, /logout, /run, /stop, /resume,
/pause, /exit, /rename, /diff, /undo, /config, /model, /models,
/provider, /doctor, /terminal-setup, /cost, /verify) fell through
to 'Unknown command'.

Implements:
- /auth status|cli|web: auth status, CLI instructions, web onboarding
- /logout <provider>: clear credentials and refresh catalog
- /run <goal>: explicit goal start
- /stop: interrupt current run
- /resume [id]: resume task or open sessions overlay
- /pause, /exit: graceful shutdown with process.exit
- /rename <name>: rename active session
- /diff: git diff --stat output
- /undo: soft reset HEAD if it's a loopcode: commit
- /config: show resolved config
- /doctor: diagnose auth, git, proxy, and environment
- /model, /models, /provider: open proxy overlay
- /cost, /verify: proper aliases for budget/verification overlays
- /mode [x]: accept explicit mode or cycle

Author: arjun-vegeta