fix(api): preserve binary response bodies

[Eotel]

Summary

bee api issues/PROJECT-1/attachments/123 > image.png now writes the attachment bytes unchanged. Previously, an HTTP 200 image response failed with a JSON parsing error and left the output empty.

1. Read the authenticated response before choosing its format. Stream non-JSON bodies to stdout; preserve JSON formatting and field selection for JSON media types.
2. Discard the body for --silent, omit output for empty responses, and report a clear error when --json is requested for a non-JSON response.
3. Add a download example to command help and regression coverage through the real SDK with fixture responses.

Closes #178

Test plan

1. The PNG regression failed before the fix with SyntaxError: ... is not valid JSON. After the fix, vp test passes 804 tests across 118 files, including all 37 API-command tests.
2. vp check, vp lint --max-warnings 0, and vp run --filter @nulab/bee generate:skill:check pass. vp run -r build builds the CLI and all 105 docs pages, with valid internal links.
3. The built CLI passes local HTTP-fixture checks on Node.js 20.20.2 and 24.21.0: the 68-byte PNG matches exactly; JSON field selection, binary --silent, the non-JSON --json error, and HTTP 204/403 handling work.

Live Backlog attachment retrieval was not tested because no target issue was supplied. Network verification used a local fixture server and dummy API-key authentication.