Phase 5: Update README with deprecation notice and archive repo

[williamzujkowski]

Parent: #66
Depends on: All nexus-agents migration phases completing (williamzujkowski/nexus-agents#643)

Objective

Once all valuable content has been absorbed into nexus-agents, update this repo's README and archive it.

Tasks

1. Update README.md with deprecation notice:

   • Clear banner: "This repository has been archived. Content has been migrated to nexus-agents."
   • Link to nexus-agents repo
   • Migration summary: what went where
   • Pointer to MIGRATION_PLAN.md for full details

2. Add migration completion notes:

   • Document what was absorbed and where it landed in nexus-agents
   • Include nexus-agents commit/PR references for each phase
   • Note what was intentionally dropped and why

3. Archive repository:

   • Set repo to read-only on GitHub
   • Ensure all branches are preserved for historical reference

Acceptance Criteria

• README has clear deprecation banner
• Migration destination map is complete
• Repo archived on GitHub (read-only)
• External references updated where possible