Update Smart Intent README docs

[thepluck]

Summary

• Replace the default Foundry README with Smart Intent protocol documentation.
• Add linked references to generated docs for core contracts, structs, hooks, and functions.
• Document router flow, hook families, base hooks, deployment scripts, and local development commands.

Impact

This gives readers a protocol-oriented entry point from the repository root while keeping detailed API references in the generated docs tree.

Validation

• git diff --check
• Verified local README Markdown link targets exist
• Checked README avoids local report references and audit-style wording

Notes

Only README.md is included in this PR. Existing local edits to .gitignore and foundry.toml were left unstaged.