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/phoenix-tracing/SKILL.md

@@ -85,10 +85,6 @@ Reference these guidelines when:
 - [fundamentals-required-attributes](references/fundamentals-required-attributes.md) - Required fields per span type
 - [fundamentals-universal-attributes](references/fundamentals-universal-attributes.md) - Common attributes (user.id, session.id)
 - [fundamentals-flattening](references/fundamentals-flattening.md) - JSON flattening rules
-- [attributes-messages](references/attributes-messages.md) - Chat message format
-- [attributes-metadata](references/attributes-metadata.md) - Custom metadata schema
-- [attributes-graph](references/attributes-graph.md) - Agent workflow attributes
-- [attributes-exceptions](references/attributes-exceptions.md) - Error tracking
 
 ## Common Workflows