fix: reveal source control item in explorer

[levivilet]

Fix the source-control item context-menu action so it switches to Explorer and reveals the selected changed file. Adds unit coverage and an end-to-end regression test for the full menu flow.

[levivilet]

CI investigation summary: the failure is deterministic on Linux, macOS, Windows, and locally. The context-menu action switches to Explorer and test.css is visible, but the selected item never receives TreeItemActive. I verified the runtime target is Explorer index 0 and the URI is the expected memfs:///workspace/test.css; merging current main does not change the failure. I also tested the existing public Explorer.revealItem command in place of Explorer.reveal, with the same result. Unit/build/static gates pass and all other E2E cases pass. The remaining bug is cross-viewlet lifecycle coordination: the reveal invoked from the Source Control command does not update Explorer focus after Source Control is replaced. This needs renderer/Explorer-level review, so I restored all diagnostic probes and marked the focused PR manual-review-needed without weakening its regression assertion.