fix(git-repo): document clobbered-branch-pointer recovery in rebase-audit

[DrumRobot]

Summary

A botched interactive rebase can leave a batch of unrelated local branches pointing at the same wrong tip (single "Created from main"-style reflog entry, no prior history). Adds a section to git-repo's rebase-audit.md covering:

• Detection (branches sharing one bogus short SHA, single-entry reflog)
• Reconciling each branch to its own GitHub PR head SHA, with content verification (merge-base diff of the PR's own changed files against the worktree's on-disk content)
• Two verification false-positive traps: mode-only diffs (Windows exec-bit loss) and "deleted file" diffs against untracked-but-present files
• Why the mixed-mode reset (never the destructive one) is the correct repair
• Why main/primary branches are a separate case (target origin/main, not any single PR head)

Test plan

• Applied this exact procedure live: 26 branches in es6kr/skills reset from a shared bogus SHA back to their verified PR head SHAs
• N/A -- documentation-only change, no code/tests affected