Task: Implement sprint-reviewer skill

Produce the sprint-reviewer skill file at doc/llm/skills/sprint-reviewer/SKILL.org and update the sprint template so every new sprint has an empty * Health Review placeholder.

• doc/llm/skills/sprint-reviewer/SKILL.org exists and is deployed.
• Skill instructs the LLM to gather data from:
  • sprint.org (mission, goals, stories table, status fields)
  • All story and task .org files in the sprint directory
  • git log --oneline --since=<sprint-start> for commit count and rate
  • gh pr list / gh pr view for PR velocity and open WIP
• Skill produces a * Health Review section with the sub-sections defined in the story acceptance: Goal alignment, Sprint load, PR velocity, Story/task balance, Focus signal, Overall verdict.
• Section is idempotent: re-running overwrites the previous review block.
• Sprint template (doc/llm/skills/new-sprint/SKILL.org or the codegen template) updated to include an empty * Health Review placeholder.

(Transient implementation strategy. Written when work starts; distilled into the parent story's * Decisions and cleared when the task closes. Plans do not outlive their task.)