feat(display): configurable tool preview length -- show full paths by default (#3841)

Tool call previews (paths, commands, queries) were hardcoded to truncate
at 35-40 chars across CLI spinners, completion lines, and gateway progress
messages. Users could not see full file paths in tool output.

New config option: display.tool_preview_length (default 0 = no limit).
Set a positive number to truncate at that length.

Changes:
- display.py: module-level _tool_preview_max_len with getter/setter;
  build_tool_preview() and get_cute_tool_message() _trunc/_path respect it
- cli.py: reads config at startup, spinner widget respects config
- gateway/run.py: reads config per-message, progress callback respects config
- run_agent.py: removed redundant 30-char quiet-mode spinner truncation
- config.py: added display.tool_preview_length to DEFAULT_CONFIG

Reported by kriskaminski

run_agent.py

@@ -5662,8 +5662,6 @@ class AIAgent:
                     face = random.choice(KawaiiSpinner.KAWAII_WAITING)
                     emoji = _get_tool_emoji(function_name)
                     preview = _build_tool_preview(function_name, function_args) or function_name
-                    if len(preview) > 30:
-                        preview = preview[:27] + "..."
                     spinner = KawaiiSpinner(f"{face} {emoji} {preview}", spinner_type='dots', print_fn=self._print_fn)
                     spinner.start()
                 _spinner_result = None