docs: fix AGENTS.md references to nonexistent CLAUDE.md

Fixes #1049 - AGENTS.md referenced CLAUDE.md which doesn't exist on main.

Changes:
- "Key Sections in CLAUDE.md" -> "Key Sections"
- "See CLAUDE.md..." -> "See AGENT_INSTRUCTIONS.md..."

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

AGENTS.md

@@ -4,7 +4,7 @@ See [AGENT_INSTRUCTIONS.md](AGENT_INSTRUCTIONS.md) for full instructions.
 
 This file exists for compatibility with tools that look for AGENTS.md.
 
-## Key Sections in CLAUDE.md
+## Key Sections
 
 - **Issue Tracking** - How to use bd for work management
 - **Development Guidelines** - Code standards and testing
@@ -18,7 +18,7 @@ This file exists for compatibility with tools that look for AGENTS.md.
 - Status: `○ ◐ ● ✓ ❄`
 - Priority: `● P0` (filled circle with color)
 
-See CLAUDE.md "Visual Design System" section for full guidance.
+See [AGENT_INSTRUCTIONS.md](AGENT_INSTRUCTIONS.md) for full development guidelines.
 
 ## Landing the Plane (Session Completion)