docs(reference): add provider capability comparison

[heimanba]

Summary

• Add provider capability and adapter implementation matrices for Bailian, Qoder, Claude, and Volcengine Ark.
• Record official documentation sources for each provider so future capability refreshes have stable starting points.
• Clarify that Claude exposes Memory Stores upstream, but the OpenAgentPack Claude adapter still marks them unsupported until implemented.
• Extend provider docs tests to keep optional adapter method rows in sync with adapter prototypes.

Evidence reviewed

• Claude API overview: Messages/Models are GA; Files, Skills, Agents, Sessions, and Environments are beta.
• Claude Managed Agents docs: skills, event streaming, scheduled deployments, multi-agent behavior, and Memory Stores are documented upstream.
• Qoder Cloud Agents API overview: gateway, beta status, headers, and resource families are documented.
• Bailian Managed Agents quickstart: /api/v1/agentstudio agents, environments, sessions, events, and SSE paths are documented.
• Volcengine Ark Managed Agents API reference: /api/v3 resource catalog reviewed from the console documentation supplied in the task.

Validation

• bun test scripts/provider-docs.test.ts
• git diff --check HEAD~1 HEAD
• git push pre-push verification profile: passed