fix(tools): install cua-driver when Computer Use is enabled via 'hermes tools' (#22765)

Returning users who enabled '🖱️ Computer Use (macOS)' via 'hermes tools'
saw '✓ Saved configuration' but no install — cua-driver was never on
PATH and the toolset failed at first use. Two compounding causes:

1. _toolset_needs_configuration_prompt fell through to _toolset_has_keys,
   which returned True for any provider with empty env_vars. cua-driver
   has no env vars, so the gate skipped _configure_toolset entirely and
   _run_post_setup('cua_driver') never ran.

2. No stable CLI entry-point existed for re-running the install when
   the picker no-op'd it (e.g. when toggling the toolset off+on inside
   one picker session, where 'added' is empty).

Changes:

- hermes_cli/tools_config.py: add _POST_SETUP_INSTALLED registry
  mapping post_setup keys to installed-state predicates. The gate
  now returns True when any visible provider has a registered
  post_setup whose predicate fails. cua_driver is the only opt-in
  for now; other post_setup hooks keep their existing behaviour.
- hermes_cli/main.py: add 'hermes computer-use install' and
  'hermes computer-use status' as a stable docs target. install
  reuses the same _run_post_setup('cua_driver') path that the
  picker invokes; status reports whether cua-driver is on PATH.
- tools/computer_use/cua_backend.py: install hint now points users
  at 'hermes computer-use install' first.
- website/docs/user-guide/features/computer-use.md: document the
  new command as the primary install path.
- website/docs/reference/cli-commands.md: catalog 'hermes
  computer-use' alongside 'hermes tools'.
- tests/hermes_cli/test_post_setup_gating.py: regression coverage
  for the gate predicate (missing -> setup forced, installed ->
  setup skipped, broken predicate -> non-blocking, unregistered
  keys -> behaviour unchanged).

Fixes #22737. Reported by @f-trycua.

tools/computer_use/cua_backend.py

@@ -84,7 +84,9 @@ def cua_driver_binary_available() -> bool:
 
 def cua_driver_install_hint() -> str:
     return (
-        "cua-driver is not installed. Install with:\n"
+        "cua-driver is not installed. Install with one of:\n"
+        "  hermes computer-use install\n"
+        "Or run the upstream installer directly:\n"
         '  /bin/bash -c "$(curl -fsSL '
         'https://raw.githubusercontent.com/trycua/cua/main/libs/cua-driver/scripts/install.sh)"\n'
         "Or run `hermes tools` and enable the Computer Use toolset to install it automatically."