refactor(skills): inline cross-links, drop Related sections, fix conventions

- all nine files: trailing "Related" sections removed, their unique links
  inlined where the topic actually arises (FFI boundary -> c-cpp, embedded
  frontend -> js-typescript, build scripts -> bash, etc.)
- README conventions: rule 2 mandated a Related Skills section, which now
  contradicts practice — replaced with inline-linking and one-home-per-fact
  rules. The <repo>/<rel path> rule was appended after the wrong section
  last commit; moved into the list where it belongs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

coding-dev.md

@@ -308,8 +308,3 @@ Full version in [`cli-tooling.md`](skills-public/cli-tooling.md); the essentials
   `nice` the build.
 - **Prefer long-running work in the background** over blocking wait loops. Never write
   `until <check>; do sleep 2; done` — it hangs the terminal and has to be interrupted by hand.
-
----
-
-## 14. Related Skills
-- [`code-languages/`](skills-public/code-languages/) — per-language rules that extend and specialize this document.