[codex] Add backend control plane inventory

[richard-devbot]

Closes #110

Summary

• Add a Node-native backend inventory scanner.
• Add rstack-agents inventory with JSON and human-readable output.
• Generate .rstack/registry/backend-inventory.json.
• Cover package assets, Pi tools/hooks/commands, CLI commands, project .rstack assets, and .pi/rstack overrides.

Validation

• node --test tests/backend-inventory.test.js
• npx eslint src/core/inventory/backend-inventory.js bin/rstack-agents.js tests/backend-inventory.test.js
• node bin/rstack-agents.js inventory --project /private/tmp/rstack-inventory-smoke --no-write
• node bin/rstack-agents.js validate

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/ble-0-inventory-110

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