Add async chatlas MCP examples

[t-kalinowski]

Summary

• Add examples/README.md and link it from the root README.
• Add async chatlas MCP examples for mcp-repl pager mode and overflow files mode using register_mcp_tools_stdio_async().
• Keep the example MCP surface to the public repl tool; the files-mode example also registers chatlas helper tools for listing output bundles and reading transcript ranges.

Public-facing changes

• New runnable chatlas examples under examples/ for Python mcp-repl sessions in both oversized-output modes.
• Example documentation now states the supported async MCP registration path and notes why there is no synchronous chat.chat() MCP example.

Internal-only changes

• Extend tests/docs_contracts.rs to lock the supported chatlas example filenames, required async registration calls, and absence of unsupported sync/prototype example paths and wording.