docs(project): create AGENTS.md for agent-oriented development

[wax911]

Summary

Creates AGENTS.md to serve as a compact instruction file for future OpenCode sessions and other AI coding tools working in this repository.

What's included

• Runtime: Deno 2.x, JSR-only (no Node.js/npm) -- the most critical guardrail
• Dev commands: Full table of deno task commands with CI-mandated ordering
• Formatting/linting: 100-char line width, double quotes, no-unused-vars excluded
• Source layout: Module-level structure with the ~1600-line monolithic CLI note
• Config resolution: 5-layer merge order with merge semantics
• Exit codes: 0-4 with meanings
• Testing conventions: FakeProcessRunner usage, co-located tests, 80% coverage minimum
• Build/release: Cross-compile targets, version bump dual-update requirement
• Branch/commit/PR conventions: Quick reference linking to CONTRIBUTING.md
• Documentation requirement: AGENTS.md and docs/ must stay updated with code changes

Every line answers: 'Would an agent likely miss this without help?'