docs: enhance README with framework matrix, comparison table, and quick start examples

[adi-singh13]

What this PR does

Transforms the README from a 7-line stub into a comprehensive, SEO-optimized landing page for the toolkit.

Changes:

• Badges — PyPI, npm, and license badges for credibility signals
• Framework matrix — Clear table showing OpenAI Agents SDK, Vercel AI SDK, and MCP support per language
• Quick start examples — Copy-paste code for both Python and TypeScript
• Capabilities section — What agents can do with the toolkit (create inboxes, send/receive, semantic search, etc.)
• Comparison table — AgentMail vs Gmail API vs SendGrid vs Resend feature matrix
• Examples links — Direct links to the best templates in agentmail-examples
• Links section — All ecosystem repos linked

Why this matters:

This is the highest-star repo (71★) in the org. The README is the first thing developers see. A rich README:

1. Increases GitHub search ranking (more keyword matches)
2. Converts visitors into users (clear value prop + copy-paste examples)
3. Captures comparison search intent ('agentmail vs resend', 'email api for agents')
4. Shows framework compatibility at a glance

SEO keywords captured:

• AI agent email
• email inbox API
• OpenAI Agents SDK email
• Vercel AI SDK email
• MCP email server
• agent framework email integration

[cubic-dev-ai]

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