improvement: add plugin adoption guide

[Luis85]

Context

Part of the plugin audit in #444 (§8 Quick Wins). There is no guide for adopters who want to install Specorator as a Claude Code plugin. Standalone mode (clone the repo) and plugin mode (install from package) have different setup steps, namespace behaviors, and hook activation paths.

Work

Create docs/plugin-adoption.md covering:

1. Standalone vs. plugin mode — what differs (file layout, CLAUDE.md loading, context budget)
2. Install command — how to install Specorator as a plugin
3. Namespace change — commands are prefixed in plugin mode: /spec:start → /specorator:spec:start
4. Hook activation — hooks in hooks/hooks.json activate automatically; hooks in .claude/settings.json do not
5. Scripts — how to invoke bin/ scripts in plugin mode
6. First-run experience — the orchestrator agent activates by default; what to expect

Definition of Done

• docs/plugin-adoption.md exists and covers all 6 sections above
• Linked from README.md install section
• Verify gate green

References

• Parent audit: research: Claude plugin developer best-practices audit — gaps, standards, refactor opportunities #444 §8
• Official install docs: https://code.claude.com/docs/en/plugins
• Hooks migration issue: improvement: migrate enforcement hooks to plugin package hooks/hooks.json #447