Add standalone agent SDK APIs and shared fund demo

[zacatac]

Summary

• sync the generated standalone-agent Developer API surface into the Python SDK
• support adding a standalone agent to a shared fund through the existing agent_tools.funds.add_user(agent_id=...) method
• document the CLI-to-SDK flow for creating an agent, granting fund access, and requesting an Agent Card credential
• add generated-client transport coverage for standalone-agent lifecycle and fund-membership operations

Test plan

• 27 public SDK tests
• Ruff formatting and lint checks
• wheel and source-distribution builds
• installed-package smoke test