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/aws-cost-optimize/SKILL.md

@@ -140,7 +140,7 @@ Wait for user confirmation before proceeding.
 **Title**: `[COST-OPT] [Resource Type] - [Brief Description] - $X/month savings`
 
 **Body**:
-```markdown
+````markdown
 ## 💰 Cost Optimization: [Brief Title]
 
 **Monthly Savings**: $X | **Risk Level**: [Low/Medium/High] | **Effort**: X days
@@ -170,7 +170,7 @@ Wait for user confirmation before proceeding.
 - [Risk and mitigation]
 
 **Priority Score**: X | **Value**: X/10 | **Risk**: X/10
-```
+````
 
 ### Step 7: Create EPIC Coordinating Issue
 **Action**: Create master tracking issue. Label with "cost-optimization" (green), "aws" (orange), "epic" (purple).