fix(workspace): clone the run's tree at repo.baseBranch, not the origin's HEAD #76

What happened

A run's working tree is checked out at the origin's HEAD, not at repo.baseBranch.
allocateWorkspace (src/lib/workspace.ts) runs git clone <mirror> <dir>, which checks out the
mirror's HEAD, the remote's default branch. baseBranch is used only by write-back, as the PR's
base.

Found during #62's live legs (#72, #67). A local bare origin created with git init --bare
may have HEAD → master, and its runs did not start from main until HEAD was pointed at
main.

Real-world effect: for a project whose PRs target a branch other than the remote's default (for
example baseBranch: "develop" on a repository whose default is main), the agent works on
main, and write-back opens a PR against develop with main's history in the diff.

Fix

Pass baseBranch into WorkspaceAllocationInput and clone with git clone --branch <baseBranch> <mirror> <dir>, or check it out after the clone. If the branch is missing, the allocation
should fail with a message that names it. Add a workspace.test.ts case with a bare origin whose
HEAD differs from baseBranch.