feat(installer): drive in-app updates through the Tauri installer

Converge update on the same principle as bootstrap: one driver owns all
repo mutation. The desktop becomes a pure consumer that hands off to
Hermes-Setup.exe --update instead of re-implementing git/pip in Electron.

- hermes desktop --build-only: build without launching, so the installer
  owns the post-update launch (CLI keeps build logic single-sourced).
- Installer AppMode {Install,Update} from argv; get_mode exposed to the UI.
- Installer self-copies to HERMES_HOME/hermes-setup.exe on install success
  (no-op guard during --update re-invocation to avoid the locked-exe copy).
- Installer --update flow (update.rs): wait for the desktop to release the
  venv shim, run 'hermes update --yes --gateway' (branch on exit 0/2/other),
  then 'hermes desktop --build-only', then launch the rebuilt desktop. Reuses
  the bootstrap event channel + progress UI via a synthetic two-stage manifest.
- Desktop applyUpdates() gutted (~105 lines of git/stash/pull/pyproject/pip
  removed) -> thin handoff: spawn updater, app.quit() to free the shim.
  Detection (checkUpdates, commit changelog, behind-count) kept intact.
- install.ps1 creates Start Menu + Desktop shortcuts to the packed Hermes.exe
  (never bare 'hermes desktop', which would rebuild every launch).

apps/bootstrap-installer/src-tauri/src/bootstrap.rs

@@ -539,6 +539,18 @@ async fn run_bootstrap(
         .unwrap_or_else(|| crate::paths::hermes_home().to_string_lossy().into_owned());
     let install_root = PathBuf::from(&hermes_home).join("hermes-agent");
 
+    // Copy ourselves to HERMES_HOME/hermes-setup.exe so the desktop app can
+    // re-invoke us with `--update` and shortcuts have a stable target. This is
+    // a one-shot install concern; an `--update` re-invocation no-ops because
+    // we're already running from that path. Best-effort — a failure here must
+    // not fail an otherwise-successful install.
+    if let Err(err) = crate::paths::copy_self_to_hermes_home() {
+        tracing::warn!(?err, "failed to copy installer into HERMES_HOME (non-fatal)");
+        emit_log(&format!(
+            "[bootstrap] warning: could not stage updater binary: {err}"
+        ));
+    }
+
     emit_event(
         &app,
         BootstrapEvent::Complete {