fix(installer): symlink bundled node/npm into command bin dir for FHS root installs

Root installs on Linux (FHS layout, #15608) put the `hermes` command in
`/usr/local/bin` (on PATH) but symlinked the bundled node/npm/npx into
`~/.local/bin`, which isn't on PATH for a stock root shell. `node`/`npm`
were 'command not found' and `hermes dashboard` failed with 'npm is not
available' because its build-on-demand fallback couldn't find npm.

Fix: `install_node()` now symlinks into `get_command_link_dir()` — the same
helper the `hermes` command link already uses — so node/npm/npx land
wherever the command does (`/usr/local/bin` on FHS root, `~/.local/bin`
otherwise, `$PREFIX/bin` on Termux). Non-root and Termux installs are
unchanged.

Also fixes:
- `scripts/lib/node-bootstrap.sh`: adds `_nb_get_link_dir()` mirroring
  the same root/Termux/user logic for the standalone bootstrap path
  (used by `hermes update`, TUI node bootstrap, etc.)
- `hermes_cli/uninstall.py`: `remove_node_symlinks()` now checks all
  candidate directories (`~/.local/bin`, `/usr/local/bin`, `$PREFIX/bin`)
  so root FHS uninstalls don't leave orphan symlinks

Regression from #15608, which created the FHS path for the command but
left `install_node` pointed at the legacy user-local dir.

tests/hermes_cli/test_uninstall_node_symlinks.py

@@ -130,3 +130,37 @@ def test_only_some_links_present(fake_home):
     assert (local_bin / "node").exists()
     assert not (local_bin / "npm").is_symlink()
     assert not (local_bin / "npx").is_symlink()
+
+
+def test_removes_fhs_symlinks_in_usr_local_bin(fake_home, tmp_path, monkeypatch):
+    """Root FHS installs place node symlinks in /usr/local/bin.
+
+    We monkeypatch _node_symlink_candidate_dirs to return a temp dir standing
+    in for /usr/local/bin so the test doesn't need real root privileges.
+    """
+    hermes_home = fake_home / ".hermes"
+    node_bin = _make_hermes_node(hermes_home)
+
+    # Fake /usr/local/bin as a temp dir with our symlinks.
+    fhs_bin = tmp_path / "usr_local_bin"
+    fhs_bin.mkdir()
+    for name in ("node", "npm", "npx"):
+        (fhs_bin / name).symlink_to(node_bin / name)
+
+    # Ensure ~/.local/bin has NO symlinks (simulate pure FHS install).
+    local_bin = fake_home / ".local" / "bin"
+    for name in ("node", "npm", "npx"):
+        p = local_bin / name
+        if p.exists() or p.is_symlink():
+            p.unlink()
+
+    # Return only our fake FHS dir as a candidate.
+    monkeypatch.setattr(
+        uninstall, "_node_symlink_candidate_dirs", lambda: [fhs_bin]
+    )
+
+    removed = uninstall.remove_node_symlinks(hermes_home)
+
+    assert sorted(p.name for p in removed) == ["node", "npm", "npx"]
+    for name in ("node", "npm", "npx"):
+        assert not (fhs_bin / name).is_symlink()