`hermes update` ran the webui build with `capture_output=True` and no timeout. On low-memory hosts (WSL2's 4 GB default, small VPSes, antivirus stalls) Vite goes silent for minutes; users see a frozen terminal, decide the update is hung, and reboot. The reboot lands *after* `pip install -e .` has already touched the install but *before* the build completes, leaving the `hermes` launcher in place while `hermes_cli` is no longer importable — i.e. `ModuleNotFoundError: No module named 'hermes_cli'` (#33788, same class as #32384).
Changes:
- New `_run_with_idle_timeout()` helper: streams subprocess output line-by-line (so the user sees Vite progress in real time) and kills the process if no bytes appear on stdout/stderr for 180s. The existing stale-dist fallback (#23817) then serves the previous build instead of failing the update.
- `_build_web_ui()` uses the helper for `npm run build` (the actual stall site). `npm install` keeps `subprocess.run` + capture_output to preserve the existing EPERM-retry-on-Windows contract.
- Both `cmd_update` call sites print `→ Core update complete. Building dashboard (optional)...` before the webui build. The CLI is fully functional at this point; a webui-build failure only affects `hermes dashboard`. Telegraphing the boundary explicitly stops users from rebooting through the build step.
Tests:
- `tests/hermes_cli/test_run_with_idle_timeout.py` — 4 tests covering streaming success, nonzero exit, idle-kill, and missing-binary cases. Uses real `subprocess.Popen` on tiny Python scripts; isolated in its own file so per-file canonical-runner parallelism doesn't pair it with the mock-heavy tests.
- `tests/hermes_cli/test_web_ui_build.py` — updated existing tests to patch `_run_with_idle_timeout` for the build step in addition to `subprocess.run` for the install step.
- `tests/hermes_cli/test_cmd_update.py::test_update_refreshes_repo_and_tui_node_dependencies` — same update.
Full suite: `scripts/run_tests.sh tests/hermes_cli/` → 5646 passed, 0 failed.
Fixes#33788.
