[BUG] MCP dependencies don't populate headers to `opencode.json`

[alopezsanchez]

Describe the bug
When a remote MCP dependency is declared in apm.yml with custom headers, apm install/apm compile doesn't populate these headers to the opencode.json

To Reproduce
Steps to reproduce the behaviour:

1. Add a remote MCP dependency to the apm.yml:

dependencies:
  mcp:
    - name: my-server
      registry: false
      transport: http
      url: "https://example.com/mymcp"
      headers:
        X-Custom-Header: "foo"

2. Run apm install/apm compile --target opencode

Expected behavior
The headers property is present in opencode.json with the values defined in apm.yml.

Environment (please complete the following information):

• OS: MacOs
• Python Version: 3.12.12
• APM Version: 0.8.11