docs(research): add pre-pivot cleanup plan
Plan to archive obsolete architecture docs, mark superseded ADRs,
remove replaced code modules (interface layer, stealth mode, control
channel), annotate stale-but-keeping docs, and create pivot spec stubs.
Key decisions:
- MPL gitserver reference archived (licensing risk + gix is the target)
- MIT/Apache rudolfs reference kept (fork candidate for git LFS)
- ADRs marked superseded, not deleted (historical record)
- Code deletion limited to modules the pivot explicitly replaces
glm-5.1
dc661dff82