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/bash.md

@@ -8,7 +8,7 @@ description: >-
 
 # Bash / Shell — Rules & Preferences
 
-Extends [`../coding-dev.md`](../coding-dev.md). Everything there applies; this adds shell specifics.
+Extends [`coding-dev.md`](skills-public/coding-dev.md). Everything there applies; this adds shell specifics.
 
 Bash is for **orchestration** — sequencing tools, wiring a build pipeline, gluing CLIs.
 Once a script grows real data structures, arithmetic, or parsing, it wants to be Python or
@@ -164,12 +164,12 @@ mistakes before a human does.
 Standalone utility scripts belong in a **dedicated, version-controlled scripts directory**,
 symlinked into `PATH` rather than copied there. Never write a loose script directly into
 `~/.local/bin` or another `PATH` directory — the source of truth should be one versioned
-location. See [`../cli-tooling.md` §5](../cli-tooling.md) for the reasoning and the pattern.
+location. See [`cli-tooling.md` §5](skills-public/cli-tooling.md) for the reasoning and the pattern.
 
 ---
 
 ## 10. Related
-- [`../coding-dev.md`](../coding-dev.md) — the general rules this extends.
-- [`../cli-tooling.md`](../cli-tooling.md) — invoking other tools *from* these scripts:
+- [`coding-dev.md`](skills-public/coding-dev.md) — the general rules this extends.
+- [`cli-tooling.md`](skills-public/cli-tooling.md) — invoking other tools *from* these scripts:
   discovery, non-interactive flags, structured output.
-- [`python.md`](./python.md) — where a shell script should have become a Python script.
+- [`python.md`](skills-public/code-languages/python.md) — where a shell script should have become a Python script.