Adopt Codex Sync identity and cross-device motivation

[ToussaintKnight]

Summary

• make Codex Sync the sole product identity and lead the README with the real cross-device continuity problem
• use $codex-sync for the Codex Skill and codexsync for the non-colliding shell CLI
• rename local protocol state, scripts, environment variables, schedulers, fixtures, and documentation with zero legacy-name residue
• mark the change as a coordinated v0.3 identity migration and warn against mixed-version fleet writers

Validation

• npm run check
• repository privacy gate: 35 files
• documentation check: 19 Markdown files / 7 language pairs
• deterministic end-to-end suite: 70 checks
• isolated global npm install and codexsync --help
• tracked-content and tracked-filename legacy-name scans: clean

Operational note

This PR updates the code repository only. The live multi-device vault remains paused and must be upgraded later as one controlled fleet maintenance event.

Closes #8