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/lib/CopyContext.py

@@ -12,9 +12,7 @@ class CopyContext:
         chroot = False
     ) -> None:
 
-        def __uri(ctx: FileContext | Uri | str) -> Uri | str | None:
-            if ctx is None:
-                return None
+        def __uri(ctx: FileContext | Uri | str) -> Uri | str:
             if isinstance(ctx, Uri):
                 return ctx
             if isinstance(ctx, str):