feat: implement acp_list_workflows tool

[jeremyeder]

Summary

Implement the acp_list_workflows MCP tool to discover available workflows from a Git repository.

Dependencies

• Requires a workflow repository endpoint or convention for discovering workflows
• Need to define the workflow definition format and discovery mechanism

API Spec

See issue #27 for the preserved API specification.

Split from #27.

[jeremyeder]

Closing — workflow tools will be tracked separately. acp_set_workflow and acp_get_workflow_metadata are landing in the scheduled sessions PR; acp_list_workflows will come in a dedicated workflows PR.