Untrack stale and misplaced docs from git

CLAUDE.md, VENDORED.md, briefs/, docs/adr/, and three working docs
(audit-fleet-prompt, perftrace-attribution-guide, tui-opentui-packaging)
no longer belong in version control. VENDORED.md was also deleted
outright since it undercounted vendored packages and is fully
superseded by docs/VENDORING.md.

CL-5905

Author: TheGreatAxios