feat(cli): two-stage /model autocomplete with ghost text suggestions (#1641)

* feat(cli): two-stage /model autocomplete with ghost text suggestions

- SlashCommandCompleter: Tab-complete providers first (anthropic:, openrouter:, etc.)
  then models within the selected provider
- SlashCommandAutoSuggest: inline ghost text for slash commands, subcommands,
  and /model provider:model two-stage suggestions
- Custom Tab key binding: accepts provider completion and immediately
  re-triggers completions to show that provider's models
- COMMANDS_BY_CATEGORY: structured format with explicit subcommands for
  tab completion and ghost text (prompt, reasoning, voice, skills, cron, browser)
- SUBCOMMANDS dict auto-extracted from command definitions
- Model/provider info cached 60s for responsive completions

* fix: repair test regression and restore gold color from PR #1622

- Fix test_unknown_command_still_shows_error: patch _cprint instead of
  console.print to match the _cprint switch in process_command()
- Restore gold color on 'Type /help' hint using _DIM + _GOLD constants
  instead of bare \033[2m (was losing the #B8860B gold)
- Use _GOLD constant for ambiguous command message for consistency
- Add clarifying comment on SUBCOMMANDS regex fallback

---------

Co-authored-by: Lars van der Zande <lmvanderzande@gmail.com>

tests/test_quick_commands.py

@@ -72,10 +72,11 @@ class TestCLIQuickCommands:
 
     def test_unknown_command_still_shows_error(self):
         cli = self._make_cli({})
-        cli.process_command("/nonexistent")
-        cli.console.print.assert_called()
-        args = cli.console.print.call_args_list[0][0][0]
-        assert "unknown command" in args.lower()
+        with patch("cli._cprint") as mock_cprint:
+            cli.process_command("/nonexistent")
+            mock_cprint.assert_called()
+            printed = " ".join(str(c) for c in mock_cprint.call_args_list)
+            assert "unknown command" in printed.lower()
 
     def test_timeout_shows_error(self):
         cli = self._make_cli({"slow": {"type": "exec", "command": "sleep 100"}})