docs: add agent guide (AGENTS.md) and remove reference recording

- AGENTS.md: project context for agents — architecture, domain facts
  (schedule math, hard mode policy, shields), build/test via Xcode MCP,
  TDD workflow expectations, UI-test harness (launch args, seed
  scenarios, accessibility identifiers, XCUI gotchas), known gaps
- CLAUDE.md points to AGENTS.md
- Delete the Opal reference screen recording (spec already captures it)
  and drop the video patterns from .gitignore

.gitignore

@@ -5,7 +5,3 @@ xcuserdata/
 # Build artifacts
 DerivedData/
 build/
-
-# Screen recordings / reference material
-*.MP4
-*.mp4