Custom OpenCode agent support

[veschin]

Problem Statement

When OpenSpec is initialized in a directory, it automatically creates markdown files configured with the build agent. However, if default agents are disabled in the configuration, OpenSpec fails to start because the build agent is unavailable. The only workaround is to manually edit each markdown file to specify a custom agent, which is inefficient and error-prone.

Reproduction Steps

1. Initialize OpenSpec in a directory using the standard initialization command.
2. Observe that markdown files are created with the default build agent configuration:

   ---
   agent: build
   description: Implement an approved OpenSpec change and keep tasks in sync.
   ---

3. Disable default agents in the OpenSpec configuration.
4. Attempt to /openspec-proposal
5. Manually edit the agent configuration in $PWD/.opencode/*.md files to a custom agent:

   ---
   description: Implement an approved OpenSpec change and keep tasks in sync.
   ---

6. Re-do /openspec-proposal

Feature Request

Add a command-line argument to specify a custom default agent during initialization.
For example:
openspec init --agent custom_agent

[TabishB]

@veschin Would just not having the agent field fix this? I think it's an optional field I believe?

[veschin]

Yes, if you remove this field, the current opencode agent will be selected.
But unfortunately openspec generates build automaticly for every new init

@veschin Would just not having the agent field fix this? I think it's an optional field I believe?

[TabishB]

@veschin ok will fix today