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/sql-code-review/SKILL.md

@@ -265,7 +265,7 @@ WHERE order_date >= '2024-01-01'
 ## 🎯 Review Output Format
 
 ### Issue Template
-```
+````
 ## [PRIORITY] [CATEGORY]: [Brief Description]
 
 **Location**: [Table/View/Procedure name and line number if applicable]
@@ -285,7 +285,7 @@ WHERE order_date >= '2024-01-01'
 ```
 
 **Expected Improvement**: [Performance gain, security benefit]
-```
+````
 
 ### Summary Assessment
 - **Security Score**: [1-10] - SQL injection protection, access controls