docs: config-management tools can roll back plugin registration

[zheng-weihao]

What changed

• USAGE §2 (Prerequisites) gains a Config-management tools subsection: provider switchers, dotfile syncers, and similar tools commonly snapshot the whole config file and restore it later, so anything written after the snapshot — including Orbit's plugin registration — is silently rolled back, while the on-disk content (marketplace snapshot, plugin cache) is never touched.
• Recovery: re-run the installer (every install re-registers and refreshes). Prevention: keep Orbit's registration entries in the tool's stored config, with the exact per-host snippets: Codex config.toml tables, Claude enabledPlugins, and the OpenCode npm plugin array (the install.sh OpenCode path needs no config entry and is unaffected).
• README install bullets link the new subsection.

Checklist

• shellcheck orbit.sh install.sh passes (N/A — docs-only change)
• bats tests/ passes (N/A — docs-only change)
• New features include tests (N/A — no behavior change)
• Documentation updated (if user-facing behavior changed)