Worktree tests miss the design's bare prunable case and two of its three mode-resolution cases #187

What's missing

The worktrees design lists specific parser and builder tests; two parts were not written.

Plan: https://github.com/ktsu-dev/GitIntegration/blob/647dbbc2dcb4e0c46cfd37568b2529f085afcc6a/docs/superpowers/specs/2026-09-21-worktrees-and-github-auth-design.md#L367-L376

Parser tests … locked and prunable each with and without a reason …

Builder tests … One per option, plus three proving the mode field resolves to the last call rather than accumulating -b alongside --detach.

What exists today

• GitIntegration.Test/Parsing/GitWorktreeParserTests.cs covers locked with and without a reason, but prunable only with a reason (ReadsAPrunableReason). GitWorktreeParser.cs (around L110-112) maps an empty value to null, but that path is untested for prunable.
• GitIntegration.Test/Builders/GitWorktreeBuilderTests.cs has one mode test, TheLastModeSelectionWins (CreatingBranch → Detached), plus a commit-ish last-wins test.

Why it matters

The mode field is last-call-wins precisely so that -b <name> and --detach never reach git together (git rejects that combination). Only one direction of one pair is tested, so a regression that, say, leaves --detach behind when CreatingBranch is called second, or emits both -b and -B, would pass the suite.

Acceptance criteria

• Parser: a bare prunable line yields IsPrunable == true and PrunableReason == null.
• Builder: two more mode-resolution tests, e.g. Detached → CreatingBranch (no --detach in the vector) and CreatingBranch → CreatingOrResettingBranch (-B present, no -b), each asserting the earlier mode's flag and operand are absent.

Dependencies

None. #141 changes the same parser (-z / quoted reasons); either can land first.