Update .env.example and README with git identity env vars #16

Summary

.env.example exists but is missing the git identity env vars (GIT_AUTHOR_* / GIT_COMMITTER_*) added to support persistent git authorship across container runs. README setup section also needs to document them.

Proposed Changes

1. Update .env.example

Add a new # Git identity section with placeholders for:

• GIT_AUTHOR_NAME
• GIT_AUTHOR_EMAIL
• GIT_COMMITTER_NAME
• GIT_COMMITTER_EMAIL

2. Update README.md

Add git identity vars to the setup section with a brief explanation of why they're needed.

Acceptance Criteria

• .env.example has git identity vars as commented placeholders
• README setup section documents the git identity vars