Improve local tooling ergonomics

This is a meta issue for improvements to the usability of our local tooling. I'd like it to cover anything achievable using `mage`, `make` and the local CI scripts. Including, but not limited to:

- Packaging
- Unit Test
- Integration tests
- Linting
- Reproducing CI failures locally in general

For reference, the kind of problem this issue is meant to include, is the fact that if I want to build a single agent package for testing locally, I need to run:

```
EXTERNAL=true SNAPSHOT=true PACKAGES=tar.gz PLATFORMS=linux/amd64 mage package
```

Why not just `mage package`? There's an even more annoying problem with integration tests. I'll file a dedicated sub-issue to deal with this.

This is a meta issue covering mage, make, and local CI scripts across packaging, unit tests, integration tests, linting, and CI-failure reproduction. Start by choosing one concrete workflow, such as the documented mage package command, and inspect the relevant existing tooling. Done should be defined by a focused sub-issue with a simpler local workflow and a verification path.

Written by the indexing model from the issue text.