test(e2e): cover tab-bar agent launches on Windows and WSL (#16110)

* test(e2e): gate the tab-bar agent launcher on Windows shells and WSL

The `+` menu agent launcher had no golden coverage in the Windows lane, so a
Windows-only break anywhere in its chain (detection row, startup-plan build,
tab create, PTY spawn, startup-command injection) could ship unnoticed.

Adds a golden spec that launches a stub agent from the menu and asserts the
agent's own banner reached the pane — a tab that spawned a bare shell instead
is indistinguishable at the store/tab layer. Runs two agents everywhere, and
on Windows also PowerShell, cmd, Git Bash and a WSL project runtime.

* test(e2e): track WSL stub agent staging state for precise cleanup

Refactor `stageWslGoldenStubAgent` to track which artifacts it creates
during setup, then only remove those artifacts during cleanup. This
prevents the test from destructively removing pre-existing symlinks or
state from previous runs, improving test isolation and idempotency.

* test(e2e): track WSL stub agent staging state for precise cleanup

- Back up and restore pre-existing stub agents to avoid destroying them
- Simplify verbose test comments to match project style guidelines

* test(e2e): serialize WSL stub agent setup with distributed lock

- Add mkdir-based lock to prevent concurrent staging invocations
- Reclaim stale locks after 10 minutes to recover from crashes
- Track lock ownership in stage state for safe cleanup

* test(e2e): track WSL stub agent staging state for precise cleanup

Track which stubs this test helper stages by writing a marker file, then
only remove stubs during stale-lock recovery if we created them. Prevents
cleanup from removing stubs left by other processes.

.github/workflows/golden-e2e-experiment.yml

@@ -74,6 +74,7 @@ jobs:
           xvfb-run --auto-servernum env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run test:e2e:terminal-rendering-golden
           xvfb-run --auto-servernum env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run --if-present test:e2e:posix-profile-index-golden
           xvfb-run --auto-servernum env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run --if-present test:e2e:agent-tui-golden
+          xvfb-run --auto-servernum env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run --if-present test:e2e:tab-bar-agent-launch-golden
           xvfb-run --auto-servernum env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run --if-present test:e2e:source-control-golden
 
       - name: Run golden E2E tests on macOS
@@ -87,6 +88,7 @@ jobs:
           env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run test:e2e:terminal-rendering-golden
           env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run --if-present test:e2e:posix-profile-index-golden
           env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run --if-present test:e2e:agent-tui-golden
+          env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run --if-present test:e2e:tab-bar-agent-launch-golden
           env SKIP_BUILD=1 ORCA_E2E_FORWARD_APP_LOGS=1 pnpm run --if-present test:e2e:source-control-golden
 
       - name: Run golden E2E tests on Windows
@@ -97,6 +99,7 @@ jobs:
           $env:ORCA_E2E_FORWARD_APP_LOGS = '1'
           pnpm run --if-present test:e2e:workspace-session-golden
           pnpm run --if-present test:e2e:windows-fresh-startup-golden
+          pnpm run --if-present test:e2e:tab-bar-agent-launch-golden
           if (Test-Path tests/e2e/golden-fresh-profile-terminal.spec.ts) {
             pnpm run test:e2e -- tests/e2e/golden-fresh-profile-terminal.spec.ts tests/e2e/golden-shell-command.spec.ts
           }