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

skills/diagnosing-superpowers/templates/case.md

@@ -0,0 +1,50 @@
+# Case: <session-id>
+
+Workspace: ~/.superpowers/diagnosing-superpowers/<session-id>/
+Created: <ISO timestamp>
+
+## Problem statement (agreed with your human partner)
+
+<One paragraph. Names the session(s), the turn range if known, what was
+expected, what happened, and the observable that matters: wall-clock,
+tokens, repeated actions, a specific unexpected action.>
+
+Goal is a superpowers bug report: yes | no
+
+## Sessions
+
+| Role | Session id | Absolute path | Lines | Bytes | Longest line (bytes) | First prompt (first 120 chars) | First timestamp |
+|---|---|---|---|---|---|---|---|
+| main | | | | | | | |
+| subagent | | | | | | | |
+
+Rejected candidates: <id — path — why rejected>, or "none".
+
+Session still running at read time: yes | no (mtime <ISO>, lines <N>)
+
+## Environment
+
+- OS: <name and version>
+- Harness: <name> <version>
+- Models seen: <model id — where (main / subagent id)>
+- Superpowers install root: <path>; version <x.y.z>; git sha <sha or "not a checkout">
+- Skill files read or injected during the session:
+
+| File (relative to install root) | sha1 (current file) | mtime newer than session? |
+|---|---|---|
+
+- Other plugins / extensions / MCP servers configured: <list, or "none found">
+- Instruction files present (paths only): <list>
+
+## Context-safety rules for every reader of these files
+
+- Check `wc -lc` and long lines (`awk '{ if (length($0) > 100000) print NR, length($0) }'`) before reading.
+- Never `cat` or `grep` for content. Line numbers and counts first
+  (`grep -n … | cut -d: -f1`), then small fields from specific lines
+  (`sed -n Np | jq -c '{…}'` or `| cut -c1-500`).
+- Read-only: never modify, move, or delete a session file.
+- In a subagent transcript, "user" is the parent agent.
+
+## Harness reference to use
+
+<references/claude-code-sessions.md | references/codex-sessions.md | references/other-harnesses.md>