docs: propose restart-safe runtime host

[danshapiro]

Summary

• document the production restart-loss evidence and current execution ownership
• compare recovery/adoption alternatives and recommend a separately supervised freshell-runtime execution plane
• specify IPC, identity, replay, service, migration, rollback, observability, and verification contracts

Verification

• FRESHELL_TEST_SUMMARY="clean origin/main before runtime-host proposal worktree" scripts/base-gate.sh test
• git diff --cached --check
• verified referenced repository paths and relative plan link