Add config.env sync tooling

[shabo]

🎯 Problem

• Keeping ~/.config/codeman/config.env aligned with the repo template was manual and error-prone (new keys get missed; old keys linger).

✅ Description

• Add config/config.env.example as the canonical template for config.env.
• Add scripts/sync_config_env.py to sync a local config file to the template while preserving user values and commented state.
• Add Makefile targets (make config-sync, make config-sync-dry, make config-sync-diff).
• Document the workflow in README.md and docs/README.md and reflect it on the GitHub Pages site.
• Add a diagram of the sync flow under docs/diagrams/.
• Ignore Python bytecode caches (__pycache__/, *.pyc).

🧭 Decision Notes

• Chose a template-preserving sync (keeps ordering/comments) to avoid churn and keep the local file readable.
• Default behavior is conservative: unknown local keys are kept unless explicitly deleted.
• Diff output is opt-in and documented as potentially secret-bearing.

🗺️ Diagrams

• docs/diagrams/config-env-sync.md (Mermaid flow of template → sync script → local config).

🧪 Validation

• Manual: python3 scripts/sync_config_env.py --help
• Manual: ./bin/codeman version

⚠️ Risk & Rollback

• Risk: make config-sync-diff can print secrets (webhook URLs) to stdout.
• Rollback: revert this PR; local user config remains unchanged unless make config-sync was run.

📈 Impact

• Easier upgrades: users can pull updates and re-sync configs safely with less manual diffing.
• Low operational risk; changes are additive and local-config tooling only.

🧠 Fun Fact 1

• GitHub CLI (gh) can create PRs directly from the terminal and will infer repo context from the current git remote.

🔬 Fun Fact 2

• The unified diff format traces back to early Unix tooling and remains a lingua franca for code review, patching, and reproducibility.