Update Decodex plugin skills

[yvette-carlisle]

Summary

• add a Decodex planning skill for issue splitting, queue shaping, dependencies, and concurrency
• route the Decodex conductor/plugin metadata to the planning skill
• update docs and skill examples to use the current multi-binary Cargo invocation

Verification

• jq . plugins/decodex/.codex-plugin/plugin.json
• git diff --check
• cargo run -p decodex --bin decodex -- --help
• cargo run -p decodex --bin decodex -- account --help
• semantic drift audit packet: no stale phrase hits or removed executable terms