fix: use raw accept header in getContent to support CODEOWNERS files > 1 MB

[Copilot]

octokit.rest.repos.getContent returns base64-encoded content capped at ~1 MB, but GitHub supports CODEOWNERS files up to 3 MB. Using the application/vnd.github.raw accept header bypasses that cap entirely (100 MB limit).

src/main.ts

• Add headers: { accept: 'application/vnd.github.raw' } to the getContent call
• With the raw header, response.data is the file content as a plain string — remove base64 decoding, encoding-type guard, and array/directory guard (directories now yield a GitHub API error caught by the existing catch block)

const response = await octokit.rest.repos.getContent({
  owner,
  repo,
  path: codeownersPath,
  ref: headSha,
  headers: {
    accept: 'application/vnd.github.raw'
  }
})
const codeownersRaw = response.data as unknown as string

__tests__/main.test.ts

• Update all getContent mocks from { data: { content: b64(x), encoding: 'base64' } } to { data: x }
• Replace the "fails when file is too large (encoding none)" test with "succeeds when the CODEOWNERS file is larger than 1 MB"
• Update the directory-path test to mock a rejected promise (GitHub returns 422 for directory paths with the raw header)
• Remove the now-unused b64() helper