feat(os): enrich instruction files with Phase 0 intake gate & sync upstream OS rules

[richfrem]

Summary

• Upstream sync from agent-plugins-skills PR #514 & PR #515.
• Enriches CLAUDE.md, GEMINI.md, AGENTS.md, and .github/copilot-instructions.md with:
  • Mandatory Phase 0 Intake & Socratic Gate (interview-spec, TASK_SPEC.md).
  • Mandatory Plugin & Skill Maintenance Policy aligned with context/plugin-config.json (fork-and-pr, local-patch-and-issue, domain-override).
• Adds dedicated fresh-clone onboarding guide INIT_AGENTS.md and links it prominently in README.md.
• Upgrades toolkit-onboarding Step 0 to interactively guide users/agents through:
  • Choosing their plugin contribution mode.
  • Initializing the Agentic OS substrate via init_agentic_os.py --retrofit.
  • Running mandatory Phase 3.5 substrate health checks (control_plane.db, hooks.json, pre-commit guard) before proceeding to account and financial baseline setup.
• Synchronizes updated ecosystem governance rules in .agent/rules/.
• Documents the complete authoring, worktree, and PR lifecycle in .agent/rules/local-worktree-and-dual-repo-edit-protocol.md.
• Validated with run_tests.py (all T0 compile, T0.5 bridge smoke, T1 unit tests, and map debt registry audits pass).