docs(handover): mirror ~/Desktop/COORD-MCP-*.md into docs/handover/

Makes the coord-MCP design log + handoff prompts + Prompt 3 handover
part of the repo so a fresh clone has the full handover context
without needing Desktop access.

- docs/handover/COORD-MCP-DESIGN-LOG.md
- docs/handover/COORD-MCP-HANDOFF-PROMPTS.md
- docs/handover/COORD-MCP-PROMPT3-HANDOVER-2026-04-20.md
- docs/handover/README.md — drift-handling policy (both copies
  updated in same commit; in-repo copy wins on conflict).

Does NOT touch the parallel Prompt 2 (007-mcp family) lane's
uncommitted work: cartridge.ncl / PROOF-SCHEDULE.adoc /
test-contracts.sh / tests/e2e_coord.ts (modified) and
.machine_readable/contractiles/bust/ (untracked).

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

Author: hyperpolymath