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/bash-agent/README.md

@@ -10,7 +10,6 @@ A bash script demonstrating how an AI agent can use bd to manage tasks autonomou
 - Random issue creation to simulate real agent behavior
 - Dependency linking with `discovered-from`
 - Statistics display
-- **Optional Agent Mail integration** for multi-agent coordination
 
 ## Prerequisites
 
@@ -21,8 +20,6 @@ A bash script demonstrating how an AI agent can use bd to manage tasks autonomou
 
 ## Usage
 
-### Basic (Single Agent)
-
 ```bash
 # Make executable
 chmod +x agent.sh
@@ -34,29 +31,6 @@ chmod +x agent.sh
 ./agent.sh 20
 ```
 
-### Multi-Agent Mode (with Agent Mail)
-
-```bash
-# Terminal 1: Start Agent Mail server
-cd ~/src/mcp_agent_mail
-source .venv/bin/activate
-python -m mcp_agent_mail.cli serve-http
-
-# Terminal 2: Run first agent
-export BEADS_AGENT_MAIL_URL=http://127.0.0.1:8765
-export BEADS_AGENT_NAME=bash-agent-1
-export BEADS_PROJECT_ID=my-project
-./agent.sh 10
-
-# Terminal 3: Run second agent (simultaneously)
-export BEADS_AGENT_MAIL_URL=http://127.0.0.1:8765
-export BEADS_AGENT_NAME=bash-agent-2
-export BEADS_PROJECT_ID=my-project
-./agent.sh 10
-```
-
-Agents will coordinate via Agent Mail to prevent claiming the same issues.
-
 ## What It Does
 
 The agent runs in a loop: