docs: document manual pin and unpin command (#474)

Update docs/reference/commands.md to describe the new pin contract
introduced in the issue #474 fix: switch <n> now pins the account for
runtime routing, best clears the pin, and unpin clears it explicitly.
Also note that a 503 codex_pinned_account_unavailable response from the
desktop app indicates the pinned account is rate-limited or otherwise
unavailable.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>