F2 SLICE-5: @vscode/test-electron host E2E (create→reload→re-anchor→orphan) (#4)

Drives the extension's returned ThreadController API + asserts the on-disk
sidecar and rendered Comments state (spec §6.8 fallback). All 4 host tests
green against VS Code 1.124.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/e2e/fixtures/workspace/docs/sample.md

@@ -0,0 +1,7 @@
+# Sample Document
+
+The quick brown fox jumps over the lazy dog.
+
+A second paragraph with the phrase anchor target inside it.
+
+Closing line.