feat: add OpenClaw provider support (#69)

[poe-code-agent]

Summary

Adds support for OpenClaw as a new AI coding agent provider, resolving #69.

Changes

• Agent definition (packages/agent-defs): Added openClawAgent definition with id, label, binary name, config path (~/.openclaw/openclaw.json), and branding colors. Exported from the registry and package index.
• Provider (src/providers/openclaw.ts): Implemented openClawService using createProvider, supporting configure (writes Poe API credentials and frontier models into the openclaw JSON config) and unconfigure (prunes the Poe provider entry). Also exports OPEN_CLAW_INSTALL_DEFINITION for npm-based installation.
• Tests (src/providers/openclaw.test.ts): 8 tests covering config creation, model selection, merging with existing config, preserving other providers, unconfigure behavior, and directory creation.
• Existing test updates: Updated packages/agent-defs/src/agents.test.ts and src/cli/isolated-services.test.ts to include openclaw in expected agent/service lists.

Behavior

• Configuring openclaw merges Poe provider credentials and frontier model list into ~/.openclaw/openclaw.json, preserving any other existing settings.
• Unconfiguring removes only the Poe provider entry; other providers (e.g., openai) are preserved.
• Returns false from unconfigure when no config file exists.