Decouple frontmatter hash computation from file system I/O

[Copilot]

Frontmatter hash computation was hardcoded to read from disk using fs.readFileSync (JavaScript) and os.ReadFile (Go). This prevented computing hashes via GitHub API or in-memory file systems.

Changes

JavaScript (actions/setup/js/frontmatter_hash_pure.cjs)

• Accept optional fileReader function in computeFrontmatterHash(options)
• Add createGitHubFileReader(github, owner, repo, ref) helper
• Default to fs.readFileSync when no custom reader provided

Go (pkg/parser/frontmatter_hash.go)

• Define FileReader type: func(filePath string) ([]byte, error)
• Add ComputeFrontmatterHashFromFileWithReader(filePath, cache, fileReader)
• Default to os.ReadFile via DefaultFileReader variable

Both implementations

• Thread custom reader through processImportsTextBased() for recursive import handling
• Fully backward compatible - existing code unchanged

Usage

// GitHub API
const reader = createGitHubFileReader(github, 'owner', 'repo', 'main');
const hash = await computeFrontmatterHash('workflow.md', { fileReader: reader });

// In-memory
const mockFS = { 'workflow.md': '---\nengine: copilot\n---\n' };
const hash = await computeFrontmatterHash('workflow.md', { 
  fileReader: async (path) => mockFS[path] 
});

// Custom reader (GitHub API, cache, etc.)
customReader := func(path string) ([]byte, error) {
    return githubClient.GetFileContent(path)
}
hash, err := parser.ComputeFrontmatterHashFromFileWithReader("workflow.md", cache, customReader)

Original prompt

Refactor frontmatter hash algorithm to use either fs module or github.getFileContent api so that it can compute the hash without files on disk.

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[github-actions]

Smoke Test Results

• ✅ Draft issue creation
• ✅ Field creation with new fields
• ✅ Field update
• ✅ Existing issue addition
• ✅ Existing PR addition
• ✅ Project status update
• ✅ Scoping validation (4 tests)

Status: PASS

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