Rewrite radkit SDK around new agent/runtime architecture

[irshadnilam]

Summary

• replace the legacy radkit crates with the new compat layer, agent primitives,
  runtime stack, tool system, and provider integrations described in radkit/spec
• refresh a2a-types and a2a-client to match the latest negotiation/streaming
  spec, including WASM runner configs and MIT licensing
• rebuild the documentation & examples: ship Astro-based docs, new end-to-end
  examples, AGENTS.md playbooks, and coverage tooling

Testing

• Not run locally (massive rewrite; follow-up runs required: cargo fmt, cargo
  clippy --all-targets --all-features, cargo test --workspace, wasm smoke tests,
  docs build)