docs(plan): add cross-repo epic guidance to plan command prompt

[acreeger]

Summary

• Adds a new "One Epic Per Repo for Cross-Repo Work" core principle to `templates/prompts/plan-prompt.txt`.
• Tells the planner to identify the repos involved, ask per-repo where each epic should be filed (trackers/projects may differ across repos), and offer two paths via AskUserQuestion:
  • Option A: create epics + children for every repo in this session (faster, contracts coordinate cross-repo).
  • Option B: create only the epics now, then re-run `il plan ` per repo (most accurate decomposition, but slower and requires more user effort).
• Reinforces: never create blocking dependencies across repos — coordinate via shared contracts in epic bodies and a sequencing recommendation in the ADR comment.

Test plan

• Manual: run `il plan` on a cross-repo feature and verify the planner now offers the two options instead of trying to fold everything into one epic.