fix: normalize deps skill names per agentskills.io spec

[phiilu]

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This PR resolves #67

I considered only deps: rules to be normalized automatically. For build: and package_skills: I opted to show a warning, because those could be fixed by the user (build:) or by the maintainer of the dependency (package_skills:).

From my project:

  defp usage_rules do
    [
      skills: [
        location: ".claude/skills",
        build: [
          phoenix_framework: [
            description: "Use this skill when working with Phoenix Framework, LiveView, Ecto, or the web layer.",
            usage_rules: [:phoenix, ~r/^phoenix_/]
          ]
        ],
        deps: [:mdex, :usage_rules]
      ]
    ]
  end

would show a warning for phoenix_framework

❯ mix usage_rules.sync

Igniter - Warnings:

* Skill name 'phoenix_framework' must only contain lowercase letters, numbers, and hyphens (agentskills.io spec)

and update the usage_rules correct:

❯ git status
On branch main
Your branch is up to date with 'origin/main'.

Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
        renamed:    .claude/skills/use-usage_rules/SKILL.md -> .claude/skills/use-usage-rules/SKILL.md
        renamed:    .claude/skills/use-usage_rules/references/elixir.md -> .claude/skills/use-usage-rules/references/elixir.md
        renamed:    .claude/skills/use-usage_rules/references/otp.md -> .claude/skills/use-usage-rules/references/otp.md
        renamed:    .claude/skills/use-usage_rules/references/usage_rules.md -> .claude/skills/use-usage-rules/references/usage_rules.md
        modified:   mix.exs

Spec reference: https://agentskills.io/specification#frontmatter-required

[zachdaniel]

The main thing we need to make sure that this does is remove any skills that used the old naming pattern. Could you add a test that shows that? i.e create an old skill using the old naming pattern w/ the old contents, and test that syncing removes that old skill? I think it will because we use frontmatter to indicate what skills we manage, but I just want to be 100% sure 😄

[phiilu]

Makes sense! I will do it as soon as possible.

[phiilu]

I added the requested test. Hopefully I did it like you intended it :)

[zachdaniel]

🚀 Thank you for your contribution! 🚀