Clean up SDK shell tool failure modes

Three concrete wraps on exec_command / write_stdin via the existing
Shell capability configure_tools mechanism, plus one skill-doc fix.
All wraps fire on both Responses and chat-completions paths; the
chat-completions error-as-result wrap still stacks on top when needed.

- write_stdin: decode the common escape forms in `chars` (\uXXXX,
  \xXX, \n \t \r \0 \a \b \v \f \\). Models routinely send the
  literal six-char string `` intending the ASCII control byte;
  the SDK takes chars verbatim so the byte never reaches the PTY and
  documented mechanisms like Ctrl-C, arrows, and Escape silently
  don't work. Allowlist regex over recognized escapes only —
  unrecognized sequences like `\p` pass through untouched.

- exec_command: catch InvalidManifestPathError and rewrite to a
  model-actionable message ("workdir must be a path inside
  /workspace") using the exception's structured `context["rel"]` so
  we don't need to string-match the SDK's wording.

- Both tools: catch pydantic ValidationError once at the wrap and
  reformat into a short "{tool}: invalid arguments — {field}: {msg}"
  string. Covers empty cmd, missing required fields, ge/min_length
  violations on max_output_tokens and yield_time_ms — and any future
  schema field the SDK adds.

Updated python.md guidance: the `shell=` parameter is for swapping
POSIX shells (bash/zsh/sh). Interpreters belong in `cmd` —
`cmd="python3 -c '...'"`, not `shell=python3`. The `shell=interpreter`
shortcut breaks in interpreter-specific ways (python needs `-c`,
node/ruby/perl need `-e`) so there's no clean code fix and we don't
try one.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

strix/skills/tooling/python.md

@@ -11,6 +11,14 @@ Prefer writing reusable scripts to `/workspace/scratch/<name>.py` and
 running them with `python3 /workspace/scratch/<name>.py`. For short
 one-off transformations, `python3 -c` or a small here-document is fine.
 
+The `shell` parameter on `exec_command` is for swapping POSIX shells
+(`bash`/`zsh`/`sh`), not for picking interpreters. Put the interpreter
+invocation in `cmd` instead: `cmd="python3 -c '...'"`, not
+`shell=python3, cmd="..."`. The `shell=<interpreter>` shortcut breaks
+in subtle ways — `python3` works only with `login=False` (because the
+SDK adds `-l`/`-i`), and other interpreters (`node`, `ruby`, `perl`)
+take `-e` not `-c` so they fail even with `login=False`.
+
 ## Proxy Automation From Python
 
 The sandbox image includes an installed `caido_api` module. Import it