feat(runtime): stream binary responses to stdout or files

[samzong]

Problem

Generated commands can receive binary responses, but pkg/runtime/client.go::doRawFullOnce always buffers the full body in memory. The output path then treats the result like ordinary formatter input; users must know to select raw output, and large downloads remain fully buffered.

This makes generated commands unsuitable for large artifacts and can corrupt terminal output when a binary response is handled through a text-oriented default formatter.

Proposal

Provide an explicit binary-response execution path based on the declared response media type or schema format.

• Stream successful response bytes directly to an explicitly selected file or stdout.
• Keep non-2xx bodies in the normal bounded error path.
• Avoid writing binary bytes to an interactive terminal without explicit user intent.
• Define overwrite behavior and never silently replace an existing file.
• Close partial files safely on cancellation or read failure.

Binary request bodies already work through --file; this issue is limited to response handling and avoiding full-response buffering.

Alternatives considered

-o raw preserves bytes but still buffers the complete response and does not provide safe file ownership or terminal behavior.

Acceptance criteria

• A large test response is copied without retaining the complete body in memory.
• stdout and file destinations preserve byte-for-byte content.
• Cancellation closes the response and leaves no successful-looking partial artifact.
• Existing JSON/table/yaml/raw behavior remains unchanged for non-binary responses.
• Catalog and generated Skill metadata explain the required output mode.

Affected surface

Runtime HTTP/auth/body/output behavior; command catalog; generated Skill docs.