chore: clean up after object dependsOn reverts

The mechanical reverts of #467 and #469 could not account for changes that
landed on main afterwards:

- #477 renamed plan task-graph snapshots from `.jsonc` to `.md`, so reverting
  the `depends_on_package_dependencies` fixture left
  `snapshots/task_graph.md` behind as an orphan with no package. The plan
  snapshot harness runs every fixture directory through workspace discovery,
  so this stale directory would fail the suite — remove it.
- `vec1` was used only by the reverted object-form `dependsOn` config, so it
  is now an unused dependency of `vite_task_graph` that `cargo shear` rejects
  — drop it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QowxsN8vDKKbQdaSMdxL67

Author: claude

Cargo.lock

@@ -18,7 +18,6 @@ serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 thiserror = { workspace = true }
 tracing = { workspace = true }
-vec1 = { workspace = true, features = ["serde"] }
 vite_path = { workspace = true }
 vite_str = { workspace = true }
 vite_workspace = { workspace = true }