Support dynamic variable resolution in config.yml

[pumpajpumpaj]

Right now (to the best of my knowledge) config.yml is fully static, which means you have to hardcode everything including local paths like projectDir. This gets annoying pretty quickly when switching between projects or sharing config with teammates who have different setups.
This would ease the transition from context providers to specific mcps, where we can store a per-project knowledge inside mcps, run different tasks on different mcps (for example, custom agent wrapped inside mcp with lsp to clean up imports without polluting main context). For now, I see the need for current workspace directory. Can expose other variables too.

It would be great to support some kind of template variable syntax that gets resolved at runtime. Something like:

mcpServers:
  - name: test
    type: streamable-http
    url: http://localhost:1234
    headers:
      projectDir: {{$workspaceDirectory}}