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/ec/ssh/AsyncSSH.py

@@ -210,9 +210,8 @@ class AsyncSSH(Base):
                         except (BrokenPipeError, OSError):
                             pass
                     return
-                if proc.stdin is None:
-                    return
-                proc.stdin.write(data)
+                if proc.stdin is not None:
+                    proc.stdin.write(data)
                 await proc.stdin.drain()
 
         async def _pump_stdout() -> None:

src/python/jw/pkg/lib/ec/ssh/util.py

@@ -42,8 +42,6 @@ def join_cmd(
     """
     ret: list[str] = []
     for token in cmd:
-        if not isinstance(token, str):
-            token = str(token)
         if token in operators:
             ret.append(token)
         else: