refactor: Remove legacy MCP Agent Mail integration (bd-6gd)

Remove the external MCP Agent Mail server integration that required
running a separate HTTP server and configuring environment variables.

The native `bd mail` system (stored as git-synced issues) remains
unchanged and is the recommended approach for inter-agent messaging.

Files removed:
- cmd/bd/message.go - Legacy `bd message` command
- integrations/beads-mcp/src/beads_mcp/mail.py, mail_tools.py
- lib/beads_mail_adapter.py - Python adapter library
- examples/go-agent/ - Agent Mail-focused example
- examples/python-agent/agent_with_mail.py, AGENT_MAIL_EXAMPLE.md
- docs/AGENT_MAIL*.md, docs/adr/002-agent-mail-integration.md
- tests/integration/test_agent_race.py, test_mail_failures.py, etc.
- tests/benchmarks/ - Agent Mail benchmarks

Updated documentation to remove Agent Mail references while keeping
native `bd mail` documentation intact.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

examples/README.md

@@ -6,7 +6,6 @@ This directory contains examples of how to integrate bd with AI agents and workf
 
 ### Agent Integration
 - **[python-agent/](python-agent/)** - Simple Python agent that discovers ready work and completes tasks
-  - **[AGENT_MAIL_EXAMPLE.md](python-agent/AGENT_MAIL_EXAMPLE.md)** - Multi-agent coordination with Agent Mail
 - **[bash-agent/](bash-agent/)** - Bash script showing the full agent workflow
 - **[startup-hooks/](startup-hooks/)** - Session startup scripts for automatic bd upgrade detection
 - **[claude-desktop-mcp/](claude-desktop-mcp/)** - MCP server for Claude Desktop integration