fix(opencode): remove invalid general-purpose agent guidance

[balloon72]

Summary

• Remove OpenCode guidance that referenced the unsupported general-purpose agent type.
• Replace missing chunk recovery with OpenCode @agent retry, smaller chunk, or serial fallback guidance.
• Avoid stopping large OpenCode runs for an interactive subfolder prompt.

Tests

• python -m pytest tests/test_install.py tests/test_install_strings.py

Notes

Closes #825

[safishamsi]

Reviewed and looks good — please mark as ready for review when you're happy with it and we'll merge.