chore: remove unused string-constant environment variables

[shenxianpeng]

Summary

Removes 8 dead-code lines at the top of main.py that define string-constant
environment variables (MESSAGE, BRANCH, AUTHOR_NAME, AUTHOR_EMAIL,
DRY_RUN, JOB_SUMMARY, PR_COMMENTS, PR_TITLE) — these were
assigned via os.getenv() but never read anywhere in the codebase.

Why safe

• The actual logic uses env_flag("MESSAGE") etc., which calls
  os.getenv() internally — the string constants were never referenced.
• log_env_vars() iterates over string literals, not these variables.

Verification

rg -w 'MESSAGE|BRANCH|AUTHOR_NAME|AUTHOR_EMAIL|DRY_RUN|JOB_SUMMARY|PR_COMMENTS|PR_TITLE' main.py

Confirmed each appears only in its definition and nowhere else. No other files
in the repo reference them either.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: db200e8c-9806-4256-bf83-8acf783ab396

📥 Commits

Reviewing files that changed from the base of the PR and between d6ad122 and 9367257.

📒 Files selected for processing (1)

• main.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/remove-unused-env-string-constants

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