Two follow-ups on commit 80f14cac (which hardened quick-branching with a
trunk fixture):
1. quick-branching.test.cjs: add a `defaultBranch` parameter to
setupFixture and run the "branches off origin/HEAD" assertion against
both `main` and `trunk`. The wholesale switch to trunk in 80f14cac
removed coverage of the conventional `main` path; parameterizing
restores it without giving up the symbolic-ref guarantee.
2. bug-2916-handle-branching-default-base.test.cjs: apply the same
parameterization here. handle_branching has the same default-branch
detection logic as Step 2.5, so it deserves the same trunk regression
guard. Previously this file only exercised `main`.
A regression that silently defaults to `main` instead of consulting
`git symbolic-ref refs/remotes/origin/HEAD` now fails the `trunk`
variant in both files.
Tests: 10/10 in the touched suites.
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.
