[arnav] clean up unused imports and dead local variables

Auto-fixed 52 ruff F401 findings (unused imports) across 22 files
in backend/ and backend/tests/. Manually resolved 8 F841 unused
locals that ruff flagged as unsafe-fix:

- agent_manager.resume_session: drop dead hours_since_closed block.
- main.py mcp-meta + outputs-meta activate handlers: drop dead
  reason = body.get("reason") binding (server ignores the field).
- dashboards.seed_demo, tools_lib.m365_device_login: keep _load(...)
  call for its 404 side-effect, drop unused binding, add intent
  comment.
- outputs.auto_run_output: keep `import anthropic` as availability
  probe, mark with `# noqa: F401` and explanation.
- dead_code_scan._extract_ws_event_branches: drop vestigial
  ws_handler_lines set (never written or read).
- test_browser_agent_unit.test_hash_tool_call_falls_back_to_repr:
  drop the unused _Boom class+instance (the actual self-referential
  bait is bad_input/bad_result; _Boom was never passed to the
  function under test).

Result: 1184/1184 backend tests pass (1 deselected: pre-existing
sandbox-only git test). ruff --select F401,F811,F841 backend/ now
clean (was 60 findings).

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/scripts/dead_code_scan.py

@@ -539,7 +539,6 @@ def _extract_ws_event_branches(tree: ast.Module) -> list[tuple[str, str]]:
     listenable events.
     """
     out: list[tuple[str, str]] = []
-    ws_handler_lines: set[int] = set()
 
     def _has_ws_decorator(fn: ast.FunctionDef | ast.AsyncFunctionDef) -> bool:
         for dec in fn.decorator_list: