feat: add diverge-critique-converge skill (#24)

## Summary

Skill for generating high-quality analysis, plans, or designs by running
a structured bakeoff. N agents independently produce solutions in
isolated worktrees, M critics review each from orthogonal angles, then
synthesize the best elements into a composite.

## New files

- `skills/diverge-critique-converge/SKILL.md`

## When to use

Triggers on: bakeoff, diverge-converge, independent analysis,
multi-perspective review, competitive analysis, adversarial review,
stress-test a plan.

Author: raiderrobert