chore: remove unused imports, dead code, and stale comments

Mechanical cleanup — no behavior changes.

Unused imports removed:
- model_tools.py: import os
- run_agent.py: OPENROUTER_MODELS_URL, get_model_context_length
- cli.py: Table, VERSION, RELEASE_DATE, resolve_toolset, get_skill_commands
- terminal_tool.py: signal, uuid, tempfile, set_interrupt_event,
  DANGEROUS_PATTERNS, _load_permanent_allowlist, _detect_dangerous_command

Dead code removed:
- toolsets.py: print_toolset_tree() (zero callers)
- browser_tool.py: _get_session_name() (never called)

Stale comments removed:
- toolsets.py: duplicated/garbled comment line
- web_tools.py: 3 aspirational TODO comments from early development

tools/browser_tool.py

@@ -614,19 +614,6 @@ def _get_session_info(task_id: Optional[str] = None) -> Dict[str, str]:
     return session_info
 
 
-def _get_session_name(task_id: Optional[str] = None) -> str:
-    """
-    Get the session name for agent-browser CLI.
-    
-    Args:
-        task_id: Unique identifier for the task
-        
-    Returns:
-        Session name for agent-browser
-    """
-    session_info = _get_session_info(task_id)
-    return session_info["session_name"]
-
 
 def _find_agent_browser() -> str:
     """