refactor: centralize Nix-managed Codex assets and integrations

modules/capabilities/ai/instructions/codex.md

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+# Environment
+
+- This is Nix; use `nix run` for missing commands. Default Nix commands to `--quiet`.
+- Keep output bounded. Capture large Nix listings in `/tmp`; inspect targeted excerpts and failing logs only. Locate store files before reading them.
+- Manage persistent binaries, skills, plugins and MCP definitions in `/home/kiri/.config/nixos`. Keep interactive settings, credentials and session state outside Nix.
+- `HERDR_ENV=1` means you are inside Herdr. Read the Herdr skill or run `herdr --skill` when Herdr interaction is needed.
+
+# Work
+
+- Inspect relevant code and callers; follow project conventions and fix root causes.
+- Prefer existing code, standard libraries and native capabilities. Avoid speculative abstractions; optimize conceptual simplicity, not line count.
+- Keep changes focused. Prefer clean breaking changes over compatibility layers or migration scripts; explain required manual steps.
+- Use targeted reads and tool calls. Verify unfamiliar or consequential APIs against version-appropriate primary documentation; prefer Context7 when needed.
+- In reviews, distinguish confirmed problems from optional improvements and explain meaningful tradeoffs.
+- Never push without my explicit approval. Other remote actions must remain within the authorized task.
+- User instructions take precedence over skill guidelines, including testing defaults.
+
+# Validation
+
+Do not add or run tests unless explicitly requested or necessary to resolve a
+concrete correctness concern. Use the smallest relevant validation and stop once
+it provides sufficient confidence. Avoid broad suites and repeated builds without
+a specific reason. Report only checks actually performed.