fix discovery support in nested dirs

[tommaso-moro]

Fixes #13372

Summary

Fix gh skill install and gh skill preview handling for nested skill paths.

What changed

• Allow --allow-hidden-dirs to discover hidden skill directories at any repository depth, not only at the root.
• Treat deeply nested skill arguments as exact repo-relative paths in both install and preview.
  • Example: packages/agent-skills/my-skill
• Preserve one-slash selectors like owner/skill as namespaced skill names, not paths.
• Share path-detection logic between install and preview.
• Add regression tests for nested hidden directories, arbitrary nested path installs, and arbitrary nested path previews.

Product note

This does not change the recommended Agent Skills publishing conventions or broaden search/publish behavior. Non-standard layouts are only supported when the user explicitly provides an exact path.

Testing

• go test ./internal/skills/discovery ./pkg/cmd/skills/install ./pkg/cmd/skills/preview -run 'TestIsSkillPath|Test_isSkillPath|TestPreviewRun|TestMatchHiddenDirConventions|TestDiscoverSkillsWithOptions|TestDiscoverLocalSkillsWithOptions|TestInstallRun' -count=1
• go test ./internal/skills/discovery ./pkg/cmd/skills/... -count=1
• git diff --check

[Copilot]

Pull request overview

Fixes gh skill install and gh skill preview behavior for nested skill layouts by improving hidden-directory discovery and by treating deeply nested selectors as repo-relative paths.

Changes:

• Extend hidden-dir discovery so --allow-hidden-dirs finds skills under hidden directories at any repository depth.
• Add shared path-detection logic (discovery.IsSkillPath) and use it to support exact nested-path installs/previews (skipping full discovery).
• Add regression tests covering nested hidden directories and arbitrary nested path selectors.

Show a summary per file

File	Description
pkg/cmd/skills/preview/preview.go	Add path-based preview flow that looks up a single skill by repo-relative path.
pkg/cmd/skills/preview/preview_test.go	Add test ensuring nested path preview skips full discovery.
pkg/cmd/skills/install/install.go	Centralize path detection via discovery.IsSkillPath.
pkg/cmd/skills/install/install_test.go	Add install test for arbitrary nested path, plus path-detection cases.
internal/skills/discovery/discovery.go	Add IsSkillPath helper and broaden hidden-dir convention matching to any depth.
internal/skills/discovery/discovery_test.go	Add coverage for nested hidden-dir matching and IsSkillPath.

Copilot's findings

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• Files reviewed: 6/6 changed files
• Comments generated: 3

[babakks]

Thanks for the PR. The implementation seems solid. Just some comments (mostly nitpicks) on tests.

[babakks]

LGTM