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>

code-languages/rust.md

@@ -96,7 +96,7 @@ workspace = true
 **Forbidden by default**, enforced in `Cargo.toml` with `unsafe_code = "forbid"`.
 
 If an FFI binding genuinely requires it, it goes behind a **safe wrapper in exactly one
-module**, with a `// SAFETY:` comment on every block stating the invariant being upheld.
+module** (see [`c-cpp.md`](skills-public/code-languages/c-cpp.md) for the other side of that boundary), with a `// SAFETY:` comment on every block stating the invariant being upheld.
 `unsafe` scattered through call sites is not acceptable.
 
 ---
@@ -189,9 +189,3 @@ throttling so a release build does not saturate the machine.
 
 **Split so that as little as possible needs the container**: a `-core` crate with no GUI
 dependencies builds and tests directly on the host, which is seconds instead of minutes.
-
----
-
-## 11. Related
-- [`coding-dev.md`](skills-public/coding-dev.md) — the general rules this extends.
-- [`c-cpp.md`](skills-public/code-languages/c-cpp.md) — when bindgen/FFI crosses into a C library.