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/python.md

@@ -9,6 +9,7 @@ description: >-
 # Python — Rules & Preferences
 
 Extends [`coding-dev.md`](skills-public/coding-dev.md). Everything there applies; this adds Python specifics.
+For the surrounding build and deploy scripts, see [`bash.md`](skills-public/code-languages/bash.md).
 
 ---
 
@@ -145,9 +146,3 @@ When talking to a rate-limited API (a chat platform, a store, a tracker):
 - **Never exercise destructive operations against real accounts or real data.** Temporary
   fixtures only.
 - `pytest -p no:randomly` only when order genuinely matters, and note why.
-
----
-
-## 10. Related
-- [`coding-dev.md`](skills-public/coding-dev.md) — the general rules this extends.
-- [`bash.md`](skills-public/code-languages/bash.md) — for the surrounding build and deploy scripts.