feat: add slash skill picker and workspace preview

[GatewayJ]

• Add read-only agent workspace APIs and shared workspace listing/preview handling.
• Reuse the workspace file tree and preview components for Hub templates and agent profile pages.
• Add direct-message slash skill selection that lists installed skills from the target agent workspace and rewrites the composer text into an explicit skill instruction.
• Tests: go test ./internal/agentworkspace ./internal/api ./internal/apitypes
• Tests: pnpm test -- workspace
• Tests: pnpm typecheck

[GatewayJ]

读取skill 目录的方式，host/mount