e2e: assert a clean tree after every command

Every mechababs transition is a `datalad run --explicit`, which saves only the declared outputs and silently leaves anything else dirty. Nothing is dirty today; this is preemptive.

A `git status --porcelain` check at the study and at the superstudy (`--ignore-submodules=none`) after each scenario step would catch a new file we forgot to declare before it lands on a real campaign, where the next iterate refuses the dirty tree and blames the operator. Cheap: one helper in the suite, run after every dispatch.

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Start in the e2e suite around the scenario-step dispatches; the issue does not name a file or test entry point. Add the shared post-dispatch checks at both the study and superstudy levels, using git status --porcelain and --ignore-submodules=none for the superstudy. Done means every scenario step verifies both trees are clean.

Written by the indexing model from the issue text.