chore: OpenCode config optimization (model, autoupdate, global AGENTS.md)

[terisuke]

Summary

Optimize local OpenCode configuration for production use.

Tasks

• Set model in opencode.jsonc
• Set small_model in opencode.jsonc
• Set autoupdate: false for source-build environment
• Create ~/.config/opencode/AGENTS.md for global agent rules
• Review and align permission rules with guardrails profile

Note

These are local config changes (not repo code), tracked for completeness.

Ref

Master Plan #51, Gap analysis report

[terisuke]

Superseded by #116 (closed). Config optimization tasks completed: model, small_model, autoupdate, AGENTS.md, instructions, formatter — 8/8.