docs: polish README for PyCon

[beonde]

Aligns README with marketing copy from capisc.io:

• Rename from 'A2A validation platform' → 'Catch bad agents before your users do'
• Add 'What It Checks' table and 'Use Cases' section
• Add Dev & Testing Commands section (badge/key/gateway are dev tools, not primary use case)
• Add npm install alternative
• Add Related Packages ecosystem section with links to Agent Guard, MCP Guard, and capiscio-core
• Streamline 'How It Works' section
• Link to capiscio-core for full CLI reference

[github-actions]

✅ All checks passed! Ready for review.

[Copilot]

Pull request overview

Updates the project README to match updated CapiscIO marketing messaging and to better separate primary CLI usage from development/testing utilities, while pointing users to capiscio-core for full CLI documentation.

Changes:

• Rewrites the README intro/positioning and streamlines the “How It Works” explanation.
• Adds “What It Checks”, “Use Cases”, and “Dev & Testing Commands” sections with examples.
• Adds an ecosystem “Related Packages” section and links to external documentation.