Review a plan too large for one review prompt, or refuse it before its items run #209

Follow-up from #201 part 1 (PR #208).

Problem. The plan schema allows a plan that does not fit the review prompt even without its file change texts: 30 items, each with 10 check: entries of up to 400 characters, plus long paths and intents. prepareReview (core/review-prompt.ts) refuses such a plan with "The plan is too large to review in one prompt…". Part 2 of #201 then sends the task to needs human, but only after every plan item has already run.

Possible fixes:

• check that the plan can be reviewed when it is imported or approved, before any item runs; or
• review such a plan in batches of items, each within the 64 KiB model budget.

Done when: a schema-valid plan that cannot be reviewed in one prompt is either refused before its items run, with a clear reason, or reviewed in batches. A test covers the largest such plan.