handle_branching in execute-phase.md (and the equivalent step in quick.md)
created the per-phase branch from whatever branch happened to be checked
out — typically the previous phase's still-unmerged feature branch — so
consecutive phases compounded on top of each other and stayed unpushed.
Detect the default branch via git symbolic-ref refs/remotes/origin/HEAD,
fast-forward it from origin, and fork the new phase branch off that tip.
Existing branches are still reused as-is. Dirty working trees fall back
to current HEAD with a loud warning, and a post-creation guard reports
any inherited commits.
Regression test extracts the bash from the <step name="handle_branching">
block structurally and runs it against a fixture repo where HEAD sits on
a previous-phase branch with extra commits.
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