[Refactor] Render E2E reproduction commands with shlex.join

Parent

Inspired by #963: use an existing standard-library implementation rather than maintaining partial quoting logic.

What to build

Make E2E reproduction-command providers return raw argv values consistently, then render a copy-pasteable shell command exactly once with shlex.join at the final presentation seam. Remove the harness's handwritten shell quote implementation and the model-owned pass-through wrappers that only duplicate shlex.quote behavior.

The current interface mixes raw tokens and pre-quoted tokens, and some final commands use a plain space join. Prompts containing spaces can therefore be rendered as different argv when copied back into a shell.

Acceptance criteria

• Every reproduction-command provider returns raw argv tokens with no pre-quoting.
• Human-readable commands are rendered once with shlex.join.
• The handwritten harness quote implementation and shallow pass-through quote wrappers are removed.
• Round-trip tests cover spaces, empty tokens, single quotes, dollar signs, backslashes, and newlines.
• Existing model-owned command selection remains model-local and the rendered commands reconstruct the original argv exactly.

Blocked by

None - can start immediately.

Start by locating the E2E reproduction-command providers, the harness's handwritten quote implementation, the model-owned pass-through wrappers, and the final command presentation seam. Trace where raw argv and pre-quoted tokens are mixed, then use the round-trip tests to verify spaces, empty tokens, quotes, dollar signs, backslashes, and newlines reconstruct the original argv exactly.