Fold delivery briefing into Decodex plugin

[yvette-carlisle]

Summary

• fold reusable delivery issue-briefing value into Decodex planning
• add Decodex-native issue briefing reference and runtime spec wording
• mark narrow lifecycle skills explicit-only to reduce active invocation budget

Verification

• cargo make fmt-rust
• cargo test -p decodex plugin_surface_tests --all-features
• git diff --check
• python3 -m json.tool docs/research/legacy-research-goal-audit.json
• python3 /Users/x/.codex/plugins/cache/hack-ink/playbook/0.3.0/scripts/semantic_drift_audit.py --repo . --json
• node /Users/x/.codex/plugins/cache/openai-curated-remote/plugin-eval/0.1.2/scripts/plugin-eval.js analyze plugins/decodex --format markdown