feat: handle symlinked plugin directories in Discover

[crossoverJie]

Summary

Discover currently skips symlinked plugin directories because
os.ReadDir entries for symlinks have ModeSymlink rather than
ModeDir, causing entry.IsDir() to return false.

This PR resolves symlinks via os.Stat so that plugin directories
placed behind a symlink are correctly discovered. Broken symlinks
produce a warning and are skipped (consistent with the existing
skip-warning behaviour for malformed plugins).

Related Issues

Checklist

Specification

• Spec changes are included in core-spec/ and follow the existing structure
• Spec changes have been discussed on the mailing list or in a linked issue
• Breaking changes to the spec are clearly called out in the summary

Ontology

• Ontology changes in ontology/ are consistent with spec changes
• New or modified terms are defined and documented

Converters

• Converter logic in converters/ is updated to reflect spec or ontology changes
• New converters include tests under the converter's test directory

Validation

• Validation rules in validation/ are updated if the spec changed
• New validation cases are covered by tests

Documentation

• docs/ is updated to reflect any user-facing changes
• New features or behaviors are documented with examples where appropriate
• CONTRIBUTING.md is updated if the contribution process changed

Examples

• examples/ are added or updated for any new spec constructs or converter support

Tests

• All existing tests pass (go test ./... / CI green)
• New functionality is covered by tests

Compliance

• ASF license headers are present on all new source files
• No third-party dependencies are added without PMC/IPMC approval

[Copilot]

Pull request overview

This PR updates the CLI’s plugin discovery to correctly treat symlinked entries that resolve to directories as discoverable plugins, aligning behavior with how users often manage plugin directories via symlinks.

Changes:

• Resolve symlinked directory entries during Discover by os.Stat-ing symlink paths and treating resolved directories as candidates.
• Emit a warning and skip entries when symlink resolution fails (e.g., broken symlinks).
• Add tests covering discovery via symlinked plugin directories and broken-symlink warning behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cli/internal/plugin/discover.go	Adds symlink resolution so symlinked plugin directories are discovered and broken symlinks warn+skip.
cli/internal/plugin/discover_test.go	Adds coverage for symlinked plugin discovery and broken symlink handling.

Comments suppressed due to low confidence (1)

cli/internal/plugin/discover_test.go:340

• This test will fail on platforms/environments where creating symlinks is not permitted (notably Windows without proper privileges). Skipping when os.Symlink fails keeps the test suite runnable while still exercising the behavior where symlinks are available.

	symlinkPath := filepath.Join(pluginsDir, "broken-link")
	if err := os.Symlink(brokenTarget, symlinkPath); err != nil {
		t.Fatalf("could not create symlink: %v", err)
	}

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