fix(install): support non-sudo service-user installs on apt distros (#25814)

The Debian/Ubuntu branch of install_node_deps() ran 'npx playwright install
--with-deps chromium' unconditionally. Playwright invokes sudo interactively
to apt-install Chromium's system libraries, which blocks the installer for
non-sudo users (systemd service accounts, unprivileged operator users) on
an unsatisfiable password prompt.

Changes:
- install.sh: gate --with-deps behind a sudo capability check on the apt
  branch (matches the existing Arch/pacman branch pattern). Non-sudo users
  fall back to 'npx playwright install chromium' alone and the installer
  prints the exact 'sudo npx playwright install-deps chromium' command an
  administrator can run separately.
- install.sh: add --skip-browser (alias --no-playwright) to skip the
  Playwright step entirely for headless installs that don't need browser
  automation. Mirrors the existing --no-venv / --skip-setup shape.
- installation.md: add a 'Non-Sudo / System Service User Installs' section
  covering the admin/service-user split, the --skip-browser flag, and the
  ~/.local/bin PATH gotcha (the root cause of the 'No module named dotenv'
  error users hit when running the repo source 'hermes' script with system
  Python instead of the venv launcher).
- test_install_sh_browser_install.py: regression coverage for the
  --skip-browser flag and the sudo-gate on the apt branch.

Reported by @ssilver in Discord.

tests/test_install_sh_browser_install.py

@@ -32,4 +32,29 @@ def test_playwright_installs_are_timeout_guarded() -> None:
 
     assert "run_browser_install_with_timeout()" in text
     assert "run_browser_install_with_timeout 600 npx playwright install chromium" in text
+    # --with-deps is still invoked on apt-based systems, but only when sudo
+    # is available non-interactively (root or passwordless sudo). Non-sudo
+    # service users fall back to the browser-only install — see
+    # install_node_deps() in install.sh.
     assert "run_browser_install_with_timeout 600 npx playwright install --with-deps chromium" in text
+
+
+def test_install_script_supports_skip_browser_flag() -> None:
+    """--skip-browser (and --no-playwright alias) skips the Playwright install."""
+    text = INSTALL_SH.read_text()
+
+    assert "--skip-browser|--no-playwright)" in text
+    assert "SKIP_BROWSER=true" in text
+    assert 'if [ "$SKIP_BROWSER" = true ]; then' in text
+    assert "--skip-browser Skip Playwright/Chromium install" in text
+
+
+def test_install_script_skips_with_deps_when_no_sudo() -> None:
+    """Non-sudo users on apt distros must not block on an interactive sudo prompt."""
+    text = INSTALL_SH.read_text()
+
+    # The apt branch must gate --with-deps behind a sudo capability check
+    # (root or non-interactive sudo), otherwise the installer hangs for
+    # service-user installs (systemd accounts, operator users, etc.).
+    assert 'if [ "$(id -u)" -eq 0 ] || (command -v sudo >/dev/null 2>&1 && sudo -n true 2>/dev/null); then' in text
+    assert "sudo npx playwright install-deps chromium" in text