Wire git worktree operations into Dispatcher/WorkflowRunner

[zeroklaw]

Background

From the review in #165: the entire git module (src/git.rs) has zero production callers. All 7 of its exported functions (clone_repo, pull_repo, create_worktree, remove_worktree, has_uncommitted_changes, sanitize_branch_name, build_clone_url) are only used in integration tests. The ~330 lines of production code and the git2 dependency exist solely for this unused module.

However, the module is clearly planned infrastructure — the module-level doc comment describes the full lifecycle (clone → pull → worktree → cleanup), GitConfig already exists in workflow.rs, and the WorkflowRunner references workflow.git.clone and workflow.git.worktree in its instructions-building logic. The actual git operations were never wired into the runner/dispatcher.

Task

Wire up the git module's worktree operations into the Dispatcher/WorkflowRunner so that when a workflow's GitConfig specifies clone and worktree settings, the runner actually performs the git clone, worktree creation/checkout, and cleanup as part of the workflow execution lifecycle.

This involves:

1. Calling build_clone_url and clone_repo/pull_repo when a dispatch arrives and workflow.git.clone is configured
2. Creating a worktree via create_worktree before running the workflow command
3. Running the workflow command inside the worktree directory
4. Cleaning up via remove_worktree after the workflow completes (success or failure)
5. Using has_uncommitted_changes for status checks as appropriate