feat: add context resource commands

[moonlight16]

Summary

• add context and contexts commands for creating, listing, reading, and deleting named resources
• support workspace, memory, knowledge, and artifacts classification metadata
• add repeatable --context NAME:/mount/path attachments to agent imports
• provide aligned table output, robust help, examples, and a small snapshot helper
• preserve existing agent-import behavior when no context is supplied

Why

Users need a concise interface for Context Service resources without writing raw HTTP requests. The attachment option connects those independently managed resources to StatefulSet and Sandbox agents through Rosso.

Dependencies

• Context Service API: feat: add named context resources context-service#1
• Rosso proxy and attachment API: feat: integrate optional Context Service rossoctl#2392

Validation

• go test ./...
• make build
• git diff --check
• manually verified rossoctl context create help and aligned rossoctl context list output

Assisted-By: Codex