feat: formalize agent adapter layer for multiple backends

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IMClaw already supports multiple agent types conceptually, but the adapter model can be made more explicit so additional backends remain maintainable.

Goals

• Define a cleaner adapter contract for agent backends
• Isolate backend-specific session and stream behavior
• Make it easier to support non-acpx backends in the future

Deliverables

• Adapter interface review
• Backend capability model
• Shared conformance tests for adapters

Acceptance criteria

• Adding a new backend does not require invasive gateway or CLI changes
• Session and stream semantics are consistent across adapters where possible