feat: add global skill uninstall flow

[Che-Zhu]

Summary

• add the global uninstall command/action and fan out UNINSTALL_SKILL tasks for covered projects
• prevent stale INSTALL_SKILL work from re-applying a skill after uninstall, while keeping stopped projects stopped
• add the /skills uninstall entry and align the skills PRDs with the implemented control flow

Testing

• pnpm exec eslint lib/skills/catalog.ts lib/platform/persistence/project-task/create-uninstall-skill-task.ts lib/platform/control/commands/skill/uninstall-global-skill.ts lib/platform/control/commands/skill/index.ts lib/actions/skill.ts lib/jobs/project-task/executors/uninstall-skill.ts lib/jobs/project-task/executors/index.ts lib/repo/project-task.ts lib/jobs/project-task/projectTaskReconcile.ts "app/(dashboard)/skills/_components/skills-library.tsx" "app/(dashboard)/skills/page.tsx"
• pnpm exec tsc --noEmit --pretty false

[github-actions]

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  Commit: c29890a016fc31a82ab72444b7d71c6fda51c7b0
  Branch: codex/global-skill-uninstall

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