fix(planning): remove Step N prefix from AddChecklist items

[aaight]

Summary

• Remove "Step N:" prefixes from checklist items generated by the planning agent
• Add explicit instruction in planning.eta that AddChecklist items should use clean task names (e.g., "Add helper function" instead of "Step 1: Add helper function")
• Add consistency note in respond-to-planning-comment.eta for checklist items added during plan updates
• Add tests verifying the no-step-prefix instructions are present in both prompts

Changes

• src/agents/prompts/templates/planning.eta: Added two explicit instructions — one in the IMPORTANT section before AddChecklist call, and one in the Rules section — instructing the LLM to omit "Step N:" prefixes from checklist items
• src/agents/prompts/templates/respond-to-planning-comment.eta: Added a note in the "Updating the Plan and Checklists" section about using clean task names without "Step N:" prefixes
• tests/unit/agents/prompts.test.ts: Added two tests verifying the no-prefix instructions exist in both prompt templates

Test plan

• npm test — all 3228 tests pass (zero failures)
• npm run typecheck — zero type errors
• npm run lint — only pre-existing warning (unrelated to this change)
• AddChecklist gadget examples already use clean names (confirmed — no changes needed)

Notes

This is a prompt-only change. The LLM (planning agent) will follow these instructions in the vast majority of cases — some rare non-compliance is acceptable and noted in the card's Risks section.

Card: https://trello.com/c/699ecb2972f1ed31177ade37

🤖 Generated with Claude Code

[nhopeatall]

✨ On it — checking the PR for that step N prefix fix

[nhopeatall]

LGTM — clean, well-scoped prompt-only change. Instructions are placed at the right locations (near AddChecklist call sites + Rules section for reinforcement), consistent across both templates, and adequately tested.