Complete Agent Mail documentation (bd-nl8z)

- Add AGENT_MAIL_QUICKSTART.md: 5-minute setup guide
- Add examples/python-agent/AGENT_MAIL_EXAMPLE.md: working code examples
- Add examples/python-agent/agent_with_mail.py: runnable multi-agent demo
- Update README.md: add Agent Mail to features and docs index
- Update AGENTS.md: enhance with quickstart/example references
- Update examples README: add Agent Mail example to index

Amp-Thread-ID: https://ampcode.com/threads/T-5d5e711d-7b5f-42ca-b75a-5b6cd843ad98
Co-authored-by: Amp <amp@ampcode.com>

examples/python-agent/README.md

@@ -84,5 +84,6 @@ tree = agent.run_bd("dep", "tree", "bd-1")
 
 ## See Also
 
+- [AGENT_MAIL_EXAMPLE.md](AGENT_MAIL_EXAMPLE.md) - Multi-agent coordination with Agent Mail
 - [../bash-agent/](../bash-agent/) - Bash version of this example
 - [../claude-desktop-mcp/](../claude-desktop-mcp/) - MCP server for Claude Desktop