Clarify agent-facing Ghost guidance

[nahiyankhan]

🤖 Created by my AI agent.

Summary

• Simplifies and clarifies Ghost docs around agent-led scan, generation, and drift workflows.
• Tightens ghost-expression and ghost-drift skill bundle guidance to describe deterministic CLI checks and authored expression.md responsibilities more plainly.
• Adds a changeset for ghost-drift and ghost-expression patch releases.

Test plan

• pnpm check via pre-commit hook
• pnpm build via pre-push hook
• pnpm check via pre-push hook
• pnpm test via pre-push hook: 34 files / 407 tests passed