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>

coding-dev.md

@@ -26,17 +26,17 @@ reasons attached, not style trivia — most exist because ignoring them caused a
 > When in doubt about which mode you are in, **ask** — it changes the answer on strictness,
 > file size, typing, and test policy all at once.
 
-Per-language specifics live in [`code-languages/`](./code-languages/):
+Per-language specifics live in [`code-languages/`](skills-public/code-languages/):
 
 | Language | Rules |
 | :--- | :--- |
-| Rust | [`code-languages/rust.md`](./code-languages/rust.md) |
-| TypeScript / JavaScript | [`code-languages/js-typescript.md`](./code-languages/js-typescript.md) |
-| Bash / Shell | [`code-languages/bash.md`](./code-languages/bash.md) |
-| Python | [`code-languages/python.md`](./code-languages/python.md) |
-| Go | [`code-languages/go.md`](./code-languages/go.md) |
-| C# / .NET | [`code-languages/csharp-dotnet.md`](./code-languages/csharp-dotnet.md) |
-| C / C++ | [`code-languages/c-cpp.md`](./code-languages/c-cpp.md) |
+| Rust | [`code-languages/rust.md`](skills-public/code-languages/rust.md) |
+| TypeScript / JavaScript | [`code-languages/js-typescript.md`](skills-public/code-languages/js-typescript.md) |
+| Bash / Shell | [`code-languages/bash.md`](skills-public/code-languages/bash.md) |
+| Python | [`code-languages/python.md`](skills-public/code-languages/python.md) |
+| Go | [`code-languages/go.md`](skills-public/code-languages/go.md) |
+| C# / .NET | [`code-languages/csharp-dotnet.md`](skills-public/code-languages/csharp-dotnet.md) |
+| C / C++ | [`code-languages/c-cpp.md`](skills-public/code-languages/c-cpp.md) |
 
 ---
 
@@ -294,7 +294,7 @@ Run the **whole** gate, not the convenient part of it. A typical full gate:
 
 ## 13. Environment & Tooling
 
-Full version in [`cli-tooling.md`](./cli-tooling.md); the essentials:
+Full version in [`cli-tooling.md`](skills-public/cli-tooling.md); the essentials:
 
 - **Read `<tool> --help` before using a CLI.** Do not guess flags, and do not write an ad-hoc
   script when an existing tool already does the job.
@@ -312,4 +312,4 @@ Full version in [`cli-tooling.md`](./cli-tooling.md); the essentials:
 ---
 
 ## 14. Related Skills
-- [`code-languages/`](./code-languages/) — per-language rules that extend and specialize this document.
+- [`code-languages/`](skills-public/code-languages/) — per-language rules that extend and specialize this document.