Composed skills generate broken reference links for deps without usage-rules.md

[Munksgaard]

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Versions

• usage_rules v1.2.4
• Elixir 1.18.4

Operating system

NixOS

Current Behavior

When using build: in the skills config with a regex to match multiple dependencies, the generated SKILL.md includes reference links for all matching deps, even those that don't ship a usage-rules.md file. This results in broken links in the "Additional References" section pointing to files that were never created.

Reproduction

Add usage_rules like the following in your mix.exs:

  defp usage_rules do
    [
      skills: [
        location: ".claude/skills",
        build: [
          "ash-framework": [
            description: """
            Use when working with Ash Framework or any Ash extension (ash_postgres, ash_oban, ash_ai, etc.).
            Consult for actions, queries, relationships, calculations, code generation, and general Ash patterns.
            For Leaf-specific patterns, also consult the ash-migrations and ash-resource-creation skills.
            """,
            usage_rules: [:ash,  ~r/^ash_/]
          ]
        ]
      ]
    ]
  end

Expected Behavior

The generated SKILL.md should only list references to files that were actually created. Deps matched by the regex but lacking a usage-rules.md file should be silently skipped in the "Additional References" section.

The "Searching Documentation" command and "Available Mix Tasks" sections can still include all matched deps (they have hexdocs and mix tasks regardless of usage-rules).