chore: complete opencode.jsonc config — instructions + formatter (8/8)

[terisuke]

Summary

Complete the remaining 2/8 opencode.jsonc configuration items.

Current State (6/8 done)

• model: anthropic/claude-sonnet-4-20250514
• small_model: anthropic/claude-haiku-4-5-20251001
• autoupdate: false
• dangerous ops deny
• .env protection
• git push control

Remaining (2/8)

• instructions: Add reference to global AGENTS.md in opencode.jsonc instructions field
• formatter: Configure formatter setting (prettier/biome) for auto-format integration

Note

These are config changes in packages/guardrails/profile/opencode.json, not local-only changes.

Ref

Master Plan #51, Issue #112