Instagram clone test: run the matrix, score, write up findings

[suleimansh]

Part of #290. Depends on #292 and #293.

• Run 2-3 repetitions per variant.
• Score every generated app against the Instagram clone test: define the spec + grading rubric #292 rubric.
• Write up: a short findings comment on Test: Instagram clone #290 (what the baseline skipped, what the anti-laziness prompt actually changed, time and cost delta) and fold the winning prompt patterns into FEATURES.md (chore: add FEATURES.md #287).

Done means we can say with evidence which automatic prompts The Framework should ship for the "make me an app" case.