feat: global agents md

[xiaoju111a]

Summary

Closes #439

Add support for global AGENTS.md file located at ~/.kimi/AGENTS.md. This allows users to define agent instructions that apply across all projects.

Changes

• Load AGENTS.md from both project directory and ~/.kimi/ directory
• Merge both files when present (project content first, global content second)
• Support both AGENTS.md and agents.md (case-insensitive)
• Add unit tests for global AGENTS.md loading and merging

Behavior

Project AGENTS.md	Global AGENTS.md	Result
✓	✓	Merged (project + global)
✓	✗	Project only
✗	✓	Global only
✗	✗	None

Usage

# Create global AGENTS.md
echo "Always use TypeScript for new files" > ~/.kimi/AGENTS.md

# Now all projects will include this instruction

Test

• test_load_agents_md_found - Project AGENTS.md exists
• test_load_agents_md_not_found - No AGENTS.md anywhere
• test_load_agents_md_lowercase - Project agents.md (lowercase)
• test_load_agents_md_global_only - Global only scenario
• test_load_agents_md_merge_project_and_global - Merge scenario
• test_load_agents_md_global_lowercase - Global lowercase support