Separate content skills from backlog backend

Decouples to-prd, to-issues, and triage from GitHub. Content skills now
own their templates and the conceptual state vocabulary; a new /github
backend skill translates (state, artifact) into gh calls.

- Rename github-triage → triage; remove all gh calls; emit handoff hints
- Create /github skill with publishPRD, publishIssues, applyTriageOutcome
- to-issues: strip gh issue create; AFK slices use AGENT-BRIEF format;
  states assigned (AFK→ready-for-agent, HITL→ready-for-human)
- to-prd: strip GitHub submit; default to ready-for-agent
- Delete triage-issue (its diagnosis half is covered by /diagnose; its
  triage half by the new /triage)
- Update plugin.json, top-level README, engineering README, deprecated README

Closes #80

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -2,9 +2,10 @@
   "name": "mattpocock-skills",
   "skills": [
     "./skills/engineering/diagnose",
+    "./skills/engineering/github",
     "./skills/engineering/grill-with-docs",
-    "./skills/engineering/github-triage",
     "./skills/engineering/improve-codebase-architecture",
+    "./skills/engineering/triage",
     "./skills/engineering/tdd",
     "./skills/engineering/to-issues",
     "./skills/engineering/to-prd",