Address CodeRabbit HIGH findings on PR #2921. The previous fix had three
unconditional code paths where `git checkout -b "$BRANCH_NAME"` would run
from the *current* HEAD when the upstream sync failed silently:
- the dirty-tree warn-and-continue path,
- the clean path where `git switch` / `git merge --ff-only` errors were
swallowed by `2>/dev/null` (still falling through to checkout -b),
- any case where `git fetch` failed but the script continued.
This rewrites both `execute-phase.md` (handle_branching) and `quick.md`
(Step 2.5) to:
1. Fetch origin/$DEFAULT_BRANCH; if fetch fails AND no local copy of
origin/$DEFAULT_BRANCH exists, abort with a clear ERROR (exit 1)
rather than create the branch off arbitrary HEAD.
2. Always create the new branch with an explicit start point:
`git checkout -b "$BRANCH_NAME" "origin/$DEFAULT_BRANCH"`. The base
is now deterministic regardless of which branch is currently
checked out, regardless of whether the optional local fast-forward
succeeded, and regardless of dirty-tree state.
3. Carry uncommitted changes onto the new (origin-pinned) branch
instead of inheriting the previous-phase HEAD as a fallback base.
The post-creation INHERITED check now references origin/$DEFAULT_BRANCH
rather than the (possibly-stale) local default branch, so the warning
fires accurately even when the local fast-forward was skipped.
4e2f1105d9