App, lib, cmds: Remove unreachable code

Remove dead code paths detected by the new warn_unreachable mypy
rule. These include:

- Removed always-false isinstance checks (ssh/util.py, templates.py)
- Removed unreachable return statements after raise (FileContext.py)
- Removed unreachable None checks for typed variables (Result.py,
  ExecContext.py, CmdGetAuthInfo.py)
- Simplified __uri function by removing impossible None check
  (CopyContext.py)
- Changed assert False to explicit error (Cmd.py)
- Removed unreachable None case from match (App.py)
- Removed redundant outer case _: pass (pkg_relations.py)
- Restructured stdin write to avoid unreachable warning (AsyncSSH.py)

Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL and pi.dev 0.81.1
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/cmds/projects/CmdGetAuthInfo.py

@@ -90,8 +90,6 @@ class CmdGetAuthInfo(Cmd):  # export
         for key, val in result.items():
             if not getattr(args, key, None):
                 continue
-            if val is None:
-                continue
             if args.only_values:
                 print(val)
                 continue

src/python/jw/pkg/cmds/projects/lib/pkg_relations.py

@@ -161,8 +161,6 @@ def pkg_relations(
                                             expanded_dep[1] = '>>'
                                         case _:
                                             pass
-                            case _:
-                                pass
                         dep_str = ' '.join(expanded_dep)
                         if quote:
                             dep_str = '"' + dep_str + '"'

src/python/jw/pkg/cmds/projects/lib/templates.py

@@ -66,10 +66,7 @@ def merge_values(*values: RenderValues) -> ListDict:
         rhs_dict = render_values_to_list_dict(rhs)
         for key, val in rhs_dict.items():
             entry = ret.setdefault(key, [])
-            if isinstance(val, list):
-                entry += val
-            else:
-                entry.append(val)
+            entry += val
     return ret
 
 def format_list_dict(