chore: remove dead code — 28 unused functions/classes across 16 files

Vulture + per-symbol verification (whole-repo grep incl. tests, string
literals, getattr, decorator/registry/argparse dispatch) confirmed each of
these has zero callers anywhere — not reachable via any dynamic-dispatch path,
not referenced by tests, not re-exported.

Removed:
- acp_adapter/tools.py: _build_patch_mode_content
- agent/anthropic_adapter.py: read_claude_managed_key (diagnostics-only, never called)
- agent/bedrock_adapter.py: get_bedrock_model_ids
- agent/browser_registry.py: get_active_browser_provider
- agent/chat_completion_helpers.py: _take_request_client (x2 nested closures, never invoked)
- gateway/platforms/weixin.py: _rewrite_headers_for_weixin, _rewrite_table_block_for_weixin
- hermes_cli/banner.py: _skin_branding
- hermes_cli/debug.py: _delete_hint
- hermes_cli/gateway.py: _setup_email, _setup_sms, _setup_yuanbao
  (platform keys absent from the _builtin_setup_fn dispatch dict; handled by
  the _setup_standard_platform fallback)
- hermes_cli/kanban_db.py: set_max_runtime, active_run
- hermes_cli/kanban_diagnostics.py: severity_of_highest, _latest_clean_event_ts
- hermes_cli/main.py: _build_provider_choices, cmd_portal
  (portal subcommand is wired via portal_cli.add_parser, not this wrapper)
- hermes_cli/model_switch.py: CustomAutoResult (orphaned by the switch_model() extraction)
- hermes_cli/models.py: format_model_pricing_table, fetch_nous_account_tier
- hermes_cli/portal_cli.py: _nous_portal_base_url
- hermes_cli/proxy/server.py: handle_models_fallback (defined but never registered on the router)
- tools/computer_use/cua_backend.py: _parse_element, _is_arm_mac
- tools/file_operations.py: _get_safe_write_root (prod uses the imported
  agent.file_safety.get_safe_write_root directly)
- tools/skills_tool.py: _load_category_description

Also dropped two imports left unused by the removals:
- tools/file_operations.py: get_safe_write_root alias
- tools/computer_use/cua_backend.py: import platform

Pure deletion: -551 LOC. No behavior change. Test files covering the edited
modules pass (640/640); the broader suite's pre-existing/env-dependent
failures reproduce unchanged on origin/main.

hermes_cli/proxy/server.py

@@ -104,17 +104,6 @@ def create_app(adapter: UpstreamAdapter) -> "web.Application":
             }
         )
 
-    async def handle_models_fallback(request: "web.Request") -> "web.Response":
-        # Most clients hit /v1/models on startup. If the upstream doesn't
-        # serve /models, synthesize a minimal response so clients don't
-        # crash. The actual forwarding path handles /models when allowed.
-        return web.json_response(
-            {
-                "object": "list",
-                "data": [],
-            }
-        )
-
     async def handle_proxy(request: "web.Request") -> "web.StreamResponse":
         # Extract the path *after* /v1
         rel_path = request.match_info.get("tail", "")