feat: add diagnosing-superpowers skill

Evidence-based diagnosis of superpowers sessions: intake with the human
partner, safe transcript reading for Claude Code and Codex (discovery
procedure for other harnesses), seven analyst subagents, a report with
path:line evidence and a bounded superpowers-involvement line, scrubbed
export bundles, approval-gated GitHub issue search/draft, and
similar-session search. Includes spec, plan, structure test, and README
and docs index lines.

Developed RED-GREEN-REFACTOR per writing-skills: 46 scored scenario runs
across five SKILL.md versions, all twelve scenarios clean against the
final version, micro-tests control 5/5 to skill 0/5 on both
baseline-failing prohibitions, and one end-to-end run. Eval records are
kept by the maintainer outside the repo.

Claude-Session: https://claude.ai/code/session_01DyaGKhTXvHNs2JgPhDktz7

README.md

@@ -294,6 +294,7 @@ Superpowers is built by [Jesse Vincent](https://blog.fsck.com) and the rest of t
 **Debugging**
 - **systematic-debugging** - 4-phase root cause process (includes root-cause-tracing, defense-in-depth, condition-based-waiting techniques)
 - **verification-before-completion** - Ensure it's actually fixed
+- **diagnosing-superpowers** - Work out what went wrong in a session, with evidence; export a scrubbed bundle or file an issue
 
 **Collaboration** 
 - **brainstorming** - Socratic design refinement