refactor: salvage adapter and CLI cleanup from PR #939

Salvaged from PR #939 by kshitij.

- deduplicate Discord slash command dispatch and local file send helpers
- deduplicate Slack file uploads while preserving thread metadata
- extract shared CLI session relative-time formatting
- hoist browser PATH cleanup constants and throttle screenshot pruning
- tidy small type and import cleanups

agent/display.py

@@ -68,7 +68,7 @@ def _oneline(text: str) -> str:
     return " ".join(text.split())
 
 
-def build_tool_preview(tool_name: str, args: dict, max_len: int = 40) -> str:
+def build_tool_preview(tool_name: str, args: dict, max_len: int = 40) -> str | None:
     """Build a short preview of a tool call's primary argument for display."""
     if not args:
         return None