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>

integrations/beads-mcp/src/beads_mcp/models.py

@@ -217,63 +217,3 @@ class InitResult(BaseModel):
     database: str
     prefix: str
     message: str
-
-
-# Agent Mail Models
-
-class MailSendParams(BaseModel):
-    """Parameters for sending mail."""
-    
-    to: list[str]
-    subject: str
-    body: str
-    urgent: bool = False
-    cc: list[str] | None = None
-    project_key: str | None = None
-    sender_name: str | None = None
-
-
-class MailInboxParams(BaseModel):
-    """Parameters for checking inbox."""
-    
-    limit: int = 20
-    urgent_only: bool = False
-    unread_only: bool = False
-    cursor: str | None = None
-    agent_name: str | None = None
-    project_key: str | None = None
-
-
-class MailReadParams(BaseModel):
-    """Parameters for reading mail."""
-    
-    message_id: int
-    mark_read: bool = True
-    agent_name: str | None = None
-    project_key: str | None = None
-
-
-class MailReplyParams(BaseModel):
-    """Parameters for replying to mail."""
-    
-    message_id: int
-    body: str
-    subject: str | None = None
-    agent_name: str | None = None
-    project_key: str | None = None
-
-
-class MailAckParams(BaseModel):
-    """Parameters for acknowledging mail."""
-    
-    message_id: int
-    agent_name: str | None = None
-    project_key: str | None = None
-
-
-class MailDeleteParams(BaseModel):
-    """Parameters for deleting mail."""
-    
-    message_id: int
-    agent_name: str | None = None
-    project_key: str | None = None