Add FTL-inspired AGENTS.md for Blits

[wouterlucas]

Adds blits/AGENTS.md modeled on ftl/AGENTS.md structure and tone, adapted for Blits contributor context.

• Drops OpenGL/Mali specifics; replaces with constrained TV/STB hardware guidance
• App-level performance rules: static/$/:-binding tiers, :for + :key + :range lazy loading, computed over template logic, .transition bindings, frameTick discipline
• Documents Component factory, template parser/codegen, reactivity, router, focus/input, engine abstraction (L3/FTL), plugins/settings/Log
• Blits equivalent of DEV guards via Log.warn/error at public boundaries
• Never-do list, review checklist, and reference file paths with docs pointers

Verified: lint 0 errors (26 pre-existing warnings), tests 1513/1513 pass.

[github-actions]

Test Results: ✅ PASSED

Run at: 2026-09-05T19:10:28.555Z

Summary:
passed: 1513 failed: 0 of 1513 tests

[github-actions]

Test Results: ✅ PASSED

Run at: 2026-09-05T19:18:15.170Z

Summary:
passed: 1370 failed: 0 of 1370 tests