fix: repair broken skill references, fence nesting, and invisible unicode (#2568)

While implementing the Agent Plugin v1 spec in https://github.com/stbenjam/skillsaw/pull/483, I used this repository as a canonical real-world source for tests. Along the way its checks surfaced genuine correctness issues worth fixing regardless of tooling:

- Internal links that do not resolve (22): wrong relative paths, links whose targets are runtime output paths rather than bundled files (now inline code), a schemeless external URL parsed as a relative path, and links to files that do not exist (removed).

- Bundled skill files never referenced from their SKILL.md (14): each is now cited from the section where it is used, so nothing ships unreachable from the skill's instructions.

- Unclosed or mis-nested code fences (11): markdown templates embedding code blocks now use four-backtick outer fences so inner fences nest instead of closing the template early and leaking example content as live markdown; also removed stray orphan fences and closed an unterminated block at end of file.

- Invisible unicode (1): removed a zero-width space from a code example, a copy-paste artifact invisible to reviewers.

skills/github-release/SKILL.md

@@ -351,7 +351,7 @@ EOF
 )"
 ```
 
-```PowerShell
+````PowerShell
 # Create PR body using here-string (preserves actual newlines, not escape sequences)
 $prBody = @"
 ## Release vX.Y.Z
@@ -367,16 +367,16 @@ This PR prepares the **vX.Y.Z** release.
 - [ ] CI passing
 
 After merging, create the tag on the merge commit:
-``````
+```
 git tag vX.Y.Z <merge-commit-sha>
 git push origin vX.Y.Z
-``````
+```
 "@
 
 # Write to file and use --body-file (do NOT use inline --body with escape sequences)
 $prBody | Out-File -FilePath release_pr_body.md -Encoding utf8 -NoNewline
 gh pr create --base main --head release/vX.Y.Z --title "Release vX.Y.Z" --body-file release_pr_body.md
-```
+````
 
 Paste the changelog section into the PR body's "What's included" block (or leave placeholder for manual review).