Sidebar tree of image references with scope toggle

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## Parent

#17

## What to build

A view that surveys a file's image references instead of requiring a line-by-line read, and that can surface a broken reference in a file the developer does not have open.

By default it lists the active file's references with their status and follows the active editor. A toggle in the view's title bar switches to workspace scope, where the top level lists values files and **expanding one is what triggers its checks** — so a repository containing many charts stays usable, and widening scope does not fire hundreds of requests at once. Current-file is the default because a repository with many charts is the normal case here, not the exception.

Clicking an entry jumps to its line in the editor. Re-check is available at entry, file, and root level, so a fix can be confirmed without waiting for a cache entry to expire.

## Acceptance criteria

- [ ] The view lists the active file's references with their status and follows the active editor
- [ ] The title-bar toggle switches between current-file and workspace scope
- [ ] Workspace scope is off by default
- [ ] In workspace scope the tree lists values files without checking them
- [ ] Expanding a file is what triggers its references to be checked
- [ ] Clicking an entry reveals its line in the editor
- [ ] Re-check works at entry, file, and root level
- [ ] View and command registration tested through the shared editor stub

## Blocked by

- #21 — Checkmark decoration and hover for image reference results
- #24 — Chart context: file scope and `appVersion`-derived tags

Start with the view and command registration and the shared editor stub mentioned in the acceptance criteria, then review issues #21 and #24. Trace how the active editor and reference checks are represented before implementing the current-file default, workspace expansion behavior, navigation, and re-check actions. Done means all listed acceptance criteria pass through the shared editor stub.

Written by the indexing model from the issue text.