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/FileContext.py

@@ -297,7 +297,6 @@ class FileContext(abc.ABC):
         except Exception as e:
             log(ERR, f'{self.log_name}: Failed to stat({path}) ({str(e)})')
             raise
-        return False
 
     async def is_dir(self, path: str, follow_symlinks = True) -> bool:
         return await self._is_dir(self._chroot(path), follow_symlinks = follow_symlinks)