Rename GitHub App workflow config vars

[Terry-Mao]

Summary

• Rename workflow GitHub App config from WORKFLOW_UPDATE_APP_CLIENT_ID / WORKFLOW_UPDATE_APP_PRIVATE_KEY to APP_CLIENT_ID / APP_PRIVATE_KEY.
• Update README setup guidance and workflow dispatch tests for the new names.
• Replace template fixture hardcodes with generic example values while preserving runtime comment markers.

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/tests
• PYTHONPYCACHEPREFIX=/tmp/aicodingflow-pycache python3 -m py_compile .github/scripts/.py .github/tests/.py
• git diff --check

Fixes #144

[github-actions]

Found: 0 critical, 1 important, 0 suggestions.

[github-actions]

No blocking issues found. The GitHub App configuration rename is reflected consistently across the workflow steps, README setup guidance, and workflow dispatch tests; no security findings were identified in the changed lines.