Previously each task created a fresh workdir via execenv.Prepare(), even
when resuming work on the same (agent, issue). This caused the agent's
session context to be out of sync with a blank code state.
Now the server returns prior_work_dir in the claim response, and the
daemon tries execenv.Reuse() first — which wraps the existing directory,
detects git worktree state, and refreshes context files. Falls back to
Prepare() if the prior workdir no longer exists. Workdirs are no longer
cleaned up after task completion so they remain available for reuse.
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