feat: add AgentMail as alternative email provider

[adi-singh13]

Summary

This PR adds AgentMail as an optional email provider alongside the existing IMAP/SMTP support. The integration is fully additive — no existing code is modified or removed.

Motivation

While IMAP/SMTP is the standard for email access, some users (especially those building AI agents) prefer a simpler setup that does not require configuring mail server hosts, ports, TLS, and credentials. AgentMail offers:

• API key auth only — no IMAP/SMTP server configuration needed
• Programmatic inbox creation — spin up new email addresses on the fly
• extracted_text — automatically strips quoted reply chains, useful for AI processing
• Simple REST API — no connection pooling, IDLE management, or reconnection logic

This makes it a natural fit as an alternative provider for an email MCP server.

What changed

New files

• src/services/agentmail.service.ts — Lightweight AgentMail API client (uses fetch, no additional npm dependencies)
• src/tools/agentmail.tool.ts — 5 MCP tools prefixed with agentmail_ to avoid name collisions

Modified files

• src/config/schema.ts — Optional [agentmail] config section with Zod validation
• src/config/loader.ts — Parse AgentMail config from TOML, updated config template
• src/types/index.ts — AgentMailConfig interface
• src/tools/register.ts — Conditionally register AgentMail tools when configured
• src/main.ts — Instantiate AgentMailService when API key is present (config or AGENTMAIL_API_KEY env var)
• README.md — Document setup, configuration, and available tools

New tools

Tool	Description
agentmail_create_inbox	Create a new inbox with a unique @agentmail.to address
agentmail_list_inboxes	List all inboxes for the API key
agentmail_list_messages	List messages in an inbox
agentmail_get_message	Get full message content
agentmail_send_message	Send email from an inbox

Configuration

Via config.toml:

[agentmail]
api_key = "your-api-key"

Or environment variable:

AGENTMAIL_API_KEY=your-api-key

Both IMAP/SMTP accounts and AgentMail can be used simultaneously — the config schema now requires at least one IMAP/SMTP account or an [agentmail] section.

Design decisions

• No new npm dependencies — the service uses native fetch to call the AgentMail REST API, avoiding any build-time dependency
• Tool name prefix — all tools use agentmail_ prefix so they coexist cleanly with existing tools
• Follows existing patterns — service/tool separation matches the codebase architecture (business logic in services, MCP wiring in tools)
• Additive only — existing IMAP/SMTP functionality is completely unchanged