bump-leg.py and setup-project.py: the git switch -c line fails "already exists" when an earlier act left that branch, and nothing says what to do (#607's sentence, in the two tools #607 left) #647

Goal: every git switch -c a tool of this standard prints says what to do when the branch already exists, in one set of words.

Filed by lane openRepoShape-5 on Brett Heap's word of 2026-10-16T11:21:49Z ("BRANCH_LEFT in bump-leg, setup"), from lane 5's checklist items on #458 (comment) and #459 (comment). Found by lane 5's #607 writer; the bump-leg line was confirmed by a probe in review rv607.

Lands after: #637 (#607), which adds the sentence BRANCH_LEFT to family.py and the holder's validate-family.py.

Lines

• bump-leg.py: with bump/spec-f1100afa319d left behind, the printed git -C <root> switch -c bump/<role>-<sha12> gives fatal: a branch named 'bump/spec-f1100afa319d' already exists, exit 128, and the remedy offers nothing more. Where: scripts/bump-leg.py _switch_command (:1124-1140 at 7ba64f0), "the one place the three switch -c lines are spelled": both refusals in _branch_to_bump_from and the dry run's closing hint (bump-leg.py --dry-run, step 1 of advancing a leg, is refused bump-leg-on-tracking-branch on every fresh project, before the old -> new line the human is to read #477).
• setup-project.py: its family next-step line git -C <holder> switch -c family/add-<project> (:2358 at 7ba64f0) is printed every time, and fails the same way where the holder still has that branch from an earlier try. L2-29 on Tracking: setup, scaffold, family and the shared helpers, creating projects and families #459 is about the same block's quoting.

Direction: the sentence #637 adds, word for word where it fits ("If git switch -c says that branch already exists, an earlier act left it: give the branch another name, which starts it where the holder stands, or, only if that branch holds work not merged yet, the same command with switch in place of switch -c goes back to it and keeps every commit on it."), with "the project" for "the holder" in bump-leg.py. Tests that type each printed line in both states, as #637's do.

Mark both lines → this issue on #458 and #459, and tick them there when this closes.

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