claude-code module: refactor to use agentapi

[johnstcn]

Note: see the https://github.com/coder/registry/tree/main/registry/coder/modules/goose module as a reference

The claude-code module was written before the agentapi module existed.
The agentapi module was extracted once we knew the required shape of a module to support multiple AI agents.

Modify the claude-code module to use the agentapi module instead of directly running it.
Additionally, add the subdomain parameter to support running cladude-code under a subdomain with agentapi version >= v0.3.3. This will cause AGENTAPI_CHAT_BASE_PATH to be exported to the correct value and be picked up by the module run script.

See the relevant PR for the goose module for reference (note that it already used the agentapi module): #299

• Modify the claude-code module to install AgentAPI and use it with claude-code
• Ensure task reporting works properly in Coder + Tasks UI
• Add a demo to show how it works with the tasks template
• Record a short video showing it working with Coder Tasks and doing status reporting