Run one extension host per remote host

[thesiti92]

Stacked PR 5 of 9 for remote reviews (base: remote/4-remote-vscode-server; #934, the rename, is merged). Review in order; each PR builds and passes its suites on its own.

Manual test plan

Purpose. Each Desktop window runs one stock extension host per remote host, inside a small per-host scope that binds files, models, diagnostics, workspace roots, search, commands and language features to their host, so hover and go to definition in a remote review answer from that host, read-only. Remote extensions are trusted the way VS Code Remote trusts them. The host's extension host reconnects as soon as main has a new endpoint, and the Rust, Swift and C# groups install on remotes.

Setup. Worktree at origin/remote/5-remote-extension-host-guard. Build the remote runtime (PR 4). R up a; R up b; R install a; R install b; R ssh a -- whiteboard remote extensions ensure (and b). Create Remote-order on a and on b. Launch; add both hosts.

Steps.

1. Each Settings row: online and Language features: available.
2. a's review → Diff view → hover one in f.ts → function one(): number.
3. Go to definition on one() → f.ts line 1, from the host; typing in it changes nothing.
4. b's review hovers too; a laptop review's hovers still work.
5. R pause b → a keeps answering; R resume b → b answers again without a reload.
6. R ssh b -- pkill -f server-main.js → b's hover answers again after the reattach.
7. Host extension commands: in the review window ⌘⇧P TypeScript lists no rows; it shows neither the laptop's nor a host's TypeScript commands. From PR 9 a Source window's palette lists the host's TypeScript commands (PR 9 step 8).
8. Optional: turn on Swift (Settings → Tools → Extensions → Manage…), Retry a → its row lists Swift with swift was not found on the login shell's PATH.

Negative checks.

• echo '{"commit":"0000000000000000000000000000000000000000"}' > apps/review-desktop/code-oss/product.overrides.json, restart Desktop → hosts stay online, reviews open, rows read Language features: unavailable — …, no hovers. Delete the file.
• Routing, one test file each: --test 'code-oss/src/vs/review/services/remote/**/*.test.ts' (the file-event filter, the scoped vscode.executeHoverProvider, models, markers, workspace roots, resolver). Journeys remote-lsp, and by name remote-lsp-rust|swift|csharp (R up --toolchain).

Cleanup. Common cleanup; make sure product.overrides.json is gone.

Reviewer notes

• Remote hosts are trusted the way VS Code Remote trusts them: a host's extension host gets what a local one gets. The per-host scope exists for routing only (upstream's URI transformer drops the authority, so files, models, diagnostics, workspace roots, search, commands and language features are bound to their host). Plan: docs/superpowers/plans/2026-10-05-adopt-vscode-trust-model.md, ruling T1.
• Not a defect: a remote extension's showOpenDialog/showSaveDialog and modal messages reach the laptop as they do in VS Code Remote (ruling T1).

Common setup for all plans

Common setup (read once)

• Test each PR in a worktree at its head branch: git -C <core> worktree add --detach ../review-prN origin/<head>, then pnpm install there. Run everything below from that worktree root.
• Helpers (bash or zsh):

R() { node apps/review-desktop/scripts/e2e/remote/remote.mjs "$@"; }
export WB_TEST_RUN=manual-$$                  # before the first `R up`
S() { ssh -F /tmp/wbt.$WB_TEST_RUN/ssh_config "$@"; }   # host aliases are wb-test-<name>
H=/tmp/wb-home-$WB_TEST_RUN                   # isolated Desktop home
mkdir -p $H/review-desktop/state/user-data/User
echo '{"review.experimental.remoteHosts.enabled": true}' > $H/review-desktop/state/user-data/User/settings.json
awk '/createRemoteReview = String.raw`/{f=1;next} /^`;/{f=0} f' apps/review-desktop/scripts/e2e/journeys/remote-host.mjs > /tmp/wb-review.sh
sed -e 's/open\\":false/open\\":true/' -e 's/wbrepo/wbrepo2/g' /tmp/wb-review.sh > /tmp/wb-push.sh
edit() { printf '{"sessionId":"%s","edit":{"type":"insert","content":{"type":"markdown","markdown":"%s"}}}' "$2" "$3" | S "$1" 'whiteboard api session_edit -'; }

• Launch (dev build only; a packaged build ignores both env vars): DEV_REVIEW_HOME=$H DEV_FAST_REVIEW_SSH_CONFIG=/tmp/wbt.$WB_TEST_RUN/ssh_config pnpm dev (prefix DEV_FAST_REVIEW_DESKTOP_BACKGROUND=1 to keep it from taking focus).
• Desktop API: D=$(ls $H/review-desktop/instances/dev-*.json); URL=$(node -p "require('$D').url"); TOK=$(node -p "require('$D').token"); api() { curl -s -H "x-review-token: $TOK" "$URL$1"; }
• A remote review: ID=$(S wb-test-a 'bash -s -- Remote-order' < /tmp/wb-review.sh | head -1) (repo ~/wbrepo, review "Remote-order" with a prose line and a code peek of f.ts). /tmp/wb-push.sh is the same with open: true in ~/wbrepo2.
• Settings: ⌘, → region Remote hosts → type the alias in SSH alias → Add.
• Fork unit tests: (cd apps/review-desktop && TSX_TSCONFIG_PATH=tsconfig.test.json node --import tsx --test --test-force-exit <files>). Package tests: pnpm --filter @dev.fast/whiteboard test <files>.
• Journeys: stage a runtime per apps/review-desktop/scripts/e2e/TESTING.md ("Staging the runtime"), then node apps/review-desktop/scripts/e2e/run.mjs --runtime "$REVIEW_E2E_RUNTIME" --journey <name>.
• The harness has pause/resume (there is no unpause).
• Cleanup, every plan: quit Desktop first (its ssh masters count as leftovers), then R down --all; R verify-clean; rm -rf $H /tmp/wb-*.sh. Without an aws sso login session verify-clean also prints AWS could not be checked and exits 1; no other line may appear.