feat: support artifact-oriented results for files, reports, and generated outputs

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Many useful agent tasks produce more than plain text. IMClaw should support structured result artifacts such as files, markdown reports, JSON outputs, and logs.

Goals

• Let requests return structured artifacts in addition to text
• Represent artifact metadata in a consistent schema
• Support downloading or retrieving stored artifacts

Deliverables

• Artifact schema
• Result transport updates
• Storage strategy for artifacts
• CLI and API support for inspecting artifacts

Acceptance criteria

• A request can return files or structured outputs as first-class results
• Clients can distinguish plain text from attached artifacts
• Artifact retrieval is stable and documented