from-plan: generic 'risk'/'risks' entries in the drop-list can discard legitimate spec sections #25

Follow-up to #24 (noticed during review, filing so it doesn't get lost).

The new leftover pass-through deliberately keeps a drop-list for reviewer-facing commentary (DROPPED_SECTIONS in plugins/cursor/scripts/lib/plan.mjs). Two entries on it are very generic: risk and risks, and isDroppedSection matches by prefix (key.startsWith(${d} )).

That means a spec's genuine sections like ## Risk mitigation or ## Risk assessment — common in RFCs and PRP-style documents, and often containing implementation-relevant constraints — are still silently discarded, which goes against the spirit of the "never drop an unmapped section" fix.

The bare Effort / risks case I care about is Claude's own plan-mode shape, so a couple of options:

1. Narrow the list to the exact plan-mode headings (effort / risks, effort/risks, open questions, …) and drop the generic risk/risks/effort prefixes.
2. Apply the drop-list only when the source is a plan-mode file (isPlanModeFile already exists) — project specs would then be fully lossless.
3. Keep the behaviour but document it in commands/from-plan.md / README.

I lean towards 2 — it matches the rule that config only applies to plan-mode files, so the same boundary would govern both the destination and the drop-list.