Issue for this PR

Closes #30409

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Addresses git identity behavior in the GitHub Action workflow without changing auth-header setup.

• Keeps existing token/header configuration logic in configureGit
• Stops overriding git author identity globally
• Adds an explicit pre-commit guard used in all commit paths:
  • pushToNewBranch
  • pushToLocalBranch
  • pushToForkBranch
• If identity is missing, fails with a clear error instead of guessing or inventing author identity
• Adds explicit prompt-context guidance for issue and PR flows to reuse configured git author identity and avoid modifying git config unless explicitly requested

How did you verify your code works?

• Verified the change scope is isolated to github/index.ts
• Ran bun typecheck in packages/opencode

Screenshots / recordings

N/A (non-UI change)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR