Release v1.16.0: install handoff skill, add publish skill, sync upstream

- Make plan2code-handoff an installed workflow prompt: move to src/,
  register in install.js SOURCE_PROMPTS, remove repo-local-only copy
- Add repo-local maintainer /plan2code-publish skill (cuts GitHub Releases;
  excluded from install.js)
- Review workflow next-step suggestion made context-aware (upstream v1.15.4)
- Doc updates: list handoff in README, QUICK-REFERENCE, architecture table;
  add "Adding a New Workflow Prompt / Skill" checklist to dev-commands
- Fix broken AGENTS.md index links: rename .agents-docs loop/metrics files
  to plan2code-* to match references
- Bump version.json to 1.16.0 to align with package.json and CHANGELOG

install.js

@@ -180,6 +180,14 @@ const SOURCE_PROMPTS = [
     name: 'finalize',
     displayName: 'Finalization Mode',
     description: 'Validate, summarize, and archive completed work'
+  },
+  {
+    source: 'plan2code-handoff.md',
+    stepNumber: 'handoff',
+    name: 'handoff',
+    displayName: 'Handoff Mode',
+    description: 'Compact the conversation into a self-contained handoff document for a fresh session',
+    isUtility: true
   }
 ];
 
@@ -200,6 +208,7 @@ function generateSkillName(prompt) {
 function generateStepLabel(prompt) {
   if (prompt.stepNumber === 'init') return 'Init';
   if (prompt.stepNumber === 'review') return 'Review';
+  if (prompt.stepNumber === 'handoff') return 'Handoff';
   return `Step ${prompt.stepNumber}`;
 }