feat: Add agents-md plugin for AGENTS.md fallback support

[baktybekb]

Summary

Adds a new plugin (plugins/agents-md) that provides AGENTS.md support for Claude Code, addressing the most upvoted issue in this repository.

• SessionStart hook detects and loads AGENTS.md files when no CLAUDE.md is present at the same directory level
• Mirrors native CLAUDE.md loading order (root-first ancestor walk)
• Supports both <dir>/AGENTS.md and <dir>/.claude/AGENTS.md locations
• Respects priority hierarchy: if CLAUDE.md exists, AGENTS.md is skipped
• Handles edge cases: special characters, unicode, large files (40k char cap)

This enables cross-tool compatibility with 20,000+ repositories using the AGENTS.md standard (Codex, Cursor, Amp, and other AI coding tools).

Resolves #6235

Behavior matrix

CLAUDE.md exists	AGENTS.md exists	Result
Yes	Yes	Only CLAUDE.md is used (native behavior)
Yes	No	Only CLAUDE.md is used (native behavior)
No	Yes	AGENTS.md is loaded by this plugin
No	No	Nothing loaded

Plugin structure

plugins/agents-md/
├── .claude-plugin/plugin.json      # Plugin manifest
├── hooks/hooks.json                # SessionStart hook config
├── hooks-handlers/session-start.sh # Detection & loading logic
└── README.md                       # Documentation

Test plan

• AGENTS.md only present — loads content correctly
• Both CLAUDE.md and AGENTS.md present — skips AGENTS.md
• Neither file present — silent, no output
• .claude/AGENTS.md nested location — loads correctly
• Mixed hierarchy (parent AGENTS.md + child CLAUDE.md) — correct per-level behavior
• Special characters, unicode, quotes, JSON-like content — safe JSON encoding

[kcrwfrd]

@baktybekb can it also support lazy loading <project_dir>/some/sub/path/AGENTS.md when reading files from that directory?