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/csharp-dotnet.md

@@ -8,7 +8,7 @@ description: >-
 
 # C# / .NET — Rules & Preferences
 
-Extends [`../coding-dev.md`](../coding-dev.md). Everything there applies; this adds .NET specifics.
+Extends [`coding-dev.md`](skills-public/coding-dev.md). Everything there applies; this adds .NET specifics.
 
 ---
 
@@ -150,5 +150,5 @@ interface, a type being renamed):
 ---
 
 ## 9. Related
-- [`../coding-dev.md`](../coding-dev.md) — the general rules this extends.
-- [`bash.md`](./bash.md) — for the release pipeline scripts around a .NET build.
+- [`coding-dev.md`](skills-public/coding-dev.md) — the general rules this extends.
+- [`bash.md`](skills-public/code-languages/bash.md) — for the release pipeline scripts around a .NET build.