It would be handy for devs to have a `harper-cli linter-tree` command

**What problem does this solve?**

For contributors to be able to see at a glance which linters are lint groups with child linters, which linters are grouped together, etc.

**Proposed Solution**

`harper-cli linter-tree` would walk the tree of `Linter`s / `LintGroup`s. Every `LintGroup` implements `Linter` and contains a collection of child `Linter`s.

But this is not exposed and probably needs a bit of internal Rust plumbing to be able to make the tree walkable.

**Examples**
Show, don't tell.

**Component**
- [x] Core engine
- [ ] Plugin/Extension
- [x] Other: `harper-cli`, `just`

Start by locating the harper-cli command dispatch and the Rust definitions of Linter and LintGroup, then inspect the related just tasks. The work is done when harper-cli linter-tree walks the linter hierarchy and clearly shows groups, child linters, and their relationships.

Written by the indexing model from the issue text.