Add `list` Command to Show Project State

[lloydrichards]

Motivation

After scaffolding with init and add, there's no quick way to see what targets and modules are present in the project. The graph command shows the catalog, not the project's state. A user returning after time away has to read the filesystem to understand what was scaffolded.

Inspiration

• Nx: nx list shows installed and available plugins
• npm: list shows installed packages with dependency tree

User Story

As a developer, I want to see which targets and modules are in my project so I can decide what to add next without guessing.

Notes

The main design question is the source of truth -- stack.effect.json doesn't track applied modules, so this likely needs filesystem scanning or a manifest. The Plan step already does filesystem scanning for conflict detection, which could be reused.

[lloydrichards]

thought: this would be quite complicated as its very possible to for users to change the name of things making it hard to use the fs to determine the existing targets and modules. could use a lock file or update the config with a history, but this would also be prone to drift