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

README.md

@@ -26,6 +26,7 @@ A structured 4-step workflow for developing features and projects with AI assist
 | `/plan2code-2-document`       | After planning, create implementation specs              |
 | `/plan2code-3-implement`      | Execute implementation (one phase per conversation)      |
 | `/plan2code-4-finalize`       | All phases complete, ready to archive                    |
+| `/plan2code-handoff`          | Compact the conversation into a handoff doc for a fresh session |
 
 **Key Rules:**
 - Start NEW conversation for each step (and each implementation phase)