feat(prompts): add llmdoc retrieval gate

[Disdjj]

Summary

• replace session-only CLI preference with an action-triggered Retrieval Gate
• route concepts, concrete files, unclear scope, known topics, and identified docs to the matching llmdoc entry point
• reapply the gate when investigation enters a new subsystem, while allowing native tools after llmdoc narrows the working set
• use the locally installed CLI through npx --no-install, sync the generated Codex skill, and update the bilingual AGENTS.md recipes
• record the durable cross-host contract in the dogfood llmdoc knowledge surface

This PR intentionally does not add a PreToolUse hook; it isolates the prompt change so its behavioral effect can be evaluated independently.

Validation

• npm run build
• npm run typecheck
• npm run lint
• npm test — 36 tests passed
• npm run check:prompts — llmdoc skill approximately 939/1600 tokens
• npx --no-install @tokenroll/llmdoc validate
• node scripts/check-codex-surface.mjs
• skill validation for canonical and generated Codex surfaces
• independent forward checks confirmed concept → search, concrete file → context --files, and subsystem pivot → a fresh Retrieval Gate