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/napkin/SKILL.md

@@ -151,4 +151,5 @@ It looks like we haven't started a napkin yet. Want me to open one for you?
 - Always check for the PNG first. If it isn't found, prompt the user to click "Share with Copilot."
 - If the clipboard doesn't have JSON data, proceed with the PNG alone.
 - The HTML template is located at `assets/napkin.html` relative to this SKILL.md file.
+- The walkthrough images `assets/step1-activate.svg`, `assets/step2-whiteboard.svg`, `assets/step3-draw.svg`, `assets/step4-share.svg`, and `assets/step5-response.svg` are documentation illustrations, not runtime inputs.
 - If the noob-mode skill is also active, use its risk indicator format (green/yellow/red) when requesting file or bash permissions.