[codex] clarify plugin design doc status

[ScriptedAlchemy]

Summary

• Clarify that docs/PLUGINS-DESIGN.md describes target plugin discovery and management behavior, not the current runtime contract.
• Point contributors at the rebrand compatibility policy when touching naming, legacy env vars, storage paths, generated config, plugin paths, or cleanup behavior.

Impact

This keeps contributor-facing documentation from implying runtime plugin discovery already exists or that legacy plugin fallback behavior is currently supported without policy review and tests.

Verification

• git diff --check
• cargo test --no-run

[changeset-bot]

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Latest commit: df702ad

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