Add an SDK-neutral agent runtime interface

[Copilot]

Rig needs to run against Copilot, Pi, Anthropic, Codex, or other SDKs without provider-specific execution paths.

Changes

• Runtime abstraction

  • Added a minimal Agent interface with ask() and close().
  • Added AgentFactory and configureAgent() for SDK adapter registration.
  • Rig creates one configured implementation per invocation.

• Copilot adapter

  • Refactored copilotEngine() into an AgentFactory.
  • Encapsulated Copilot session creation, messaging, events, and cleanup.

• SDK-neutral APIs

  • Replaced addon session access with context.agent.
  • Renamed oncePerSession() to oncePerAgent().
  • Decoupled tool definitions from Copilot SDK types.

configureAgent(async ({ model, systemMessage, tools }) =>
  new MySdkAgent({ model, systemMessage, tools })
);

[pelikhan]

@copilot import agent sdk for

• pi-agent
• Anthropic sdk

[Copilot]

@copilot import agent sdk for

• pi-agent...

Added pi-agent and Anthropic SDK adapters with tool mapping, stateful turns, cancellation/error handling, tests, and usage docs in 6290080.

[pelikhan]

@copilot Add tests

[Copilot]

@copilot Add tests

Added focused pi-agent and Anthropic adapter tests in 45961a0, covering cancellation, validation, defaults, and mixed response content.