feat(pstack): add /in-your-own-words to play back the goal before work

[poteto]

Summary

Adds /in-your-own-words to pstack. It runs only when typed (disable-model-invocation: true). Before any work, the agent plays back in its own plain words what the user wants and the problem underneath it, marks its guesses, and stops so the user can correct a misreading before the run starts.

• The play-back uses up to four parts (Goal, Problem, Assumptions, Not sure), stays under 120 words, and keeps exact names and values such as IDs, amounts, limits, and dates.
• It proposes no plan or fix, but it may name a fix or option that already exists in what the user shared.
• It treats the user's theory of the cause as a hypothesis. When the evidence already rules that theory out, the agent states the evidence instead of asking whether to make the change anyway.
• It asks at most one question, only when the answer changes the work, and says what each answer would change.

The skill is also listed in the pstack README and in the help guide's skill table and prompt recipes.

Test plan

• node scripts/validate-plugins.mjs passes.