feat(cli): Add upload and download commands for file transfer

[iiilisan]

Summary

Add aviato upload and aviato download CLI commands for transferring files to/from sandboxes, wrapping the existing SDK methods sandbox.write_file() and sandbox.read_file().

Motivation

The SDK already has read_file() / write_file() methods but there's no CLI equivalent. Users currently have to use aviato exec <id> sh -c "echo ... > /path" as a workaround for file transfer, which is error-prone and limited to small payloads.

PR #60 review (NavarrePratt) suggested dedicated commands.

Proposed commands

aviato upload <sandbox-id> <local-path> <remote-path>
aviato download <sandbox-id> <remote-path> [local-path]

• upload: read local file, call sandbox.write_file(remote_path, content)
• download: call sandbox.read_file(remote_path), write to local path (default: stdout or basename)

Acceptance criteria

• aviato upload transfers a local file into a sandbox
• aviato download retrieves a file from a sandbox
• Both handle missing sandbox (SandboxNotFoundError) via existing _AviatoCLI handler
• Both handle missing file (SandboxFileError) via existing _AviatoCLI handler
• Unit tests with mocked Sandbox
• Docs updated (AGENTS.md CLI table)