docs: use <repo>/<rel path> links throughout, document the convention

All cross-skill links now spell out the repository: skills-public/foo.md,
skills-private/bar.md. These resolve from the shared parent directory, so one
spelling works from either repo and from code-languages/ subdirectories —
replacing the previous mix of ./, ../ and absolute file:// URLs. Link labels
normalised to match. Convention added to the README conventions list.

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

code-languages/c-cpp.md

@@ -8,7 +8,7 @@ description: >-
 
 # C / C++ — Rules & Preferences
 
-Extends [`../coding-dev.md`](../coding-dev.md). Everything there applies; this adds C/C++ specifics.
+Extends [`coding-dev.md`](skills-public/coding-dev.md). Everything there applies; this adds C/C++ specifics.
 
 C and C++ are for the cases with no better option: existing codebases, hard real-time paths,
 and FFI boundaries. **For new standalone work, prefer Rust** — the memory-safety guarantees
@@ -148,6 +148,6 @@ When exposing C/C++ to another language:
 ---
 
 ## 9. Related
-- [`../coding-dev.md`](../coding-dev.md) — the general rules this extends.
-- [`rust.md`](./rust.md) — the preferred language for new native work, and the usual consumer
+- [`coding-dev.md`](skills-public/coding-dev.md) — the general rules this extends.
+- [`rust.md`](skills-public/code-languages/rust.md) — the preferred language for new native work, and the usual consumer
   of these FFI boundaries.