feat: expose HybridClaw as an MCP server or standardized external agent API

[furukama]

Summary

HybridClaw has strong MCP client support and its own internal web/API surfaces, but it does not appear to expose a first-class server-side integration surface for external editors or agent clients such as an MCP server endpoint or a standardized agent API.

Why

This would let other tools treat HybridClaw as a reusable agent runtime instead of only a standalone CLI/gateway product. It would also make editor and automation integrations much easier.

Proposed scope

• Evaluate two target surfaces:
  1. HybridClaw as an MCP server
  2. HybridClaw as a standardized external agent API (for example OpenAI-style chat/responses-compatible subset)
• Choose one as the first shipped interface and define the compatibility boundary clearly.
• Expose core operations such as send message, stream response, list/search sessions, inspect agents, and optionally manage attachments.
• Define auth and permission boundaries for remote clients.

Candidate MVP

• HTTP MCP server endpoint or stdio MCP mode backed by the existing gateway runtime
• tools/resources for conversations, sessions, agents, and message submission
• token-protected API surface with explicit scope controls

Implementation notes

• Do not mirror every internal concept on day one; start with a narrow, stable contract.
• Reuse existing gateway auth, session routing, and audit primitives where possible.
• Treat remote clients as untrusted and preserve the current approval/security model.
• Document the compatibility story clearly so users know whether the goal is MCP-first, OpenAI-compatible, or both.

Acceptance criteria

• External clients can connect to HybridClaw through one documented, supported server-side interface.
• The interface supports at least message submission and response retrieval/streaming.
• Auth, audit, and approval behavior are preserved for remote callers.
• The docs include setup examples and a supported capability matrix.
• Tests cover auth failures, basic request flow, and one end-to-end client integration path.