feat(opencode): load repo root AGENTS for external reads

[edmundmiller]

Summary\n- load only the repo-root AGENTS.md when reading files outside the active project\n- keep existing in-project progressive loading behavior unchanged\n- add regression coverage for external repo reads\n\n## Testing\n- cd packages/opencode && bun test test/session/instruction.test.ts\n- cd packages/opencode && bun test test/tool/read.test.ts -t "loaded instructions"\n- cd packages/opencode && bun typecheck

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The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found related PRs that address similar functionality:

Potentially Related PRs:

1. PR feat(opencode): load .opencode/AGENTS.md instructions #12096 - feat(opencode): load .opencode/AGENTS.md instructions

   • Directly related to AGENTS loading functionality

2. PR fix(read): skip external directory prompt for global AGENTS.md #13753 - fix(read): skip external directory prompt for global AGENTS.md

   • Addresses external reads with AGENTS.md handling

3. PR feat: support .local.md variants for instruction files #16108 - feat: support .local.md variants for instruction files

   • Related to instruction file loading patterns

These PRs are related to the same area (AGENTS/instructions loading) but don't appear to be exact duplicates. PR #16920 specifically focuses on loading repo-root AGENTS for external reads, which appears to be a focused improvement over the previous approaches in these related PRs.