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>

cli-tooling.md

@@ -11,7 +11,7 @@ description: >-
 # CLI & Tooling — Discovery, Invocation, and Script Placement
 
 How to use command-line tools correctly, and where utility scripts belong.
-Companion to [`coding-dev.md`](./coding-dev.md), which covers writing the code itself.
+Companion to [`coding-dev.md`](skills-public/coding-dev.md), which covers writing the code itself.
 
 ---
 
@@ -62,7 +62,7 @@ Anything run from a script, a CI job, or an agent must not depend on a human bei
 - **Use the tool's own blocking flag** (`--wait`, `--follow`, `--no-detach`) when you need an
   operation to finish before the next step. Do not build a polling loop around a command that
   already knows how to wait — and never write a shell `until`/`while` wait loop
-  (see [`code-languages/bash.md`](./code-languages/bash.md)).
+  (see [`code-languages/bash.md`](skills-public/code-languages/bash.md)).
 - **Prefer a `--dry-run` rehearsal** for anything that deploys, releases, or deletes, when the
   tool offers one.
 - Set `--yes` / `--non-interactive` deliberately and only where the action is genuinely safe —
@@ -91,7 +91,7 @@ tool list -o csv        # structured columns for a spreadsheet or script
 > [!WARNING]
 > **Never pipe build, lint, or test output through a filter to make it shorter.** That is a
 > different situation and the rule there is the opposite — see
-> [`coding-dev.md` §2](./coding-dev.md). Structured output flags are for data; `grep` on
+> [`coding-dev.md` §2](skills-public/coding-dev.md). Structured output flags are for data; `grep` on
 > diagnostics hides warnings.
 
 ---
@@ -184,6 +184,6 @@ system, not scattered at the repository root.
 ---
 
 ## 8. Related Skills
-- [`coding-dev.md`](./coding-dev.md) — writing the code these tools build and test.
-- [`code-languages/bash.md`](./code-languages/bash.md) — writing the scripts themselves:
+- [`coding-dev.md`](skills-public/coding-dev.md) — writing the code these tools build and test.
+- [`code-languages/bash.md`](skills-public/code-languages/bash.md) — writing the scripts themselves:
   strict mode, quoting, build-gate structure, destructive-command guards.