docs: define gradual Shell-to-Go migration architecture

[SakuraIsayeki]

Summary

• define the target Go core with language-agnostic executable driver and exporter boundaries
• freeze the current Shell plugin behavior as protocol v1 so the Go core can run existing plugins unchanged
• describe phased policy centralization, selective plugin rewrites, compatibility and rollback rules, release gates, and restore-based verification
• keep Shell plugins supported with no removal milestone in this migration plan
• link the architecture from the README

Validation

• checked the proposal against the current CLI, plugin paths and verbs, manifest schema 2, staging behavior, Debian packages, and systemd unit
• verified the branch is two commits ahead of main and changes only documentation

No runtime behavior is changed by this pull request.