feat: reuse a failed herdr coder dispatch's pane for one retry instead of a fresh one

Added

• A herdr coder dispatch that fails verification now retries in the same pane instead of a
  fresh one, once per issue. Every dispatch's tab used to close the moment its result came
  back, win or lose, so a fixable-verdict retry always started a brand-new agent process —
  the previous attempt's context (files already read, decisions already made) was thrown
  away every round. dispatchViaHerdr now leaves a successful call's pane open via
  spec.herdrPersistPane (returned as herdrTabId/herdrPaneId) and, given
  spec.herdrReuse, skips workspace/tab-create/agent-start entirely and reissues agent prompt straight into that pane. handleVerificationFailure queues the pane for exactly
  one reuse via Sprint's new markHerdrReusePending/consumeHerdrReusePending/
  herdrReuseState, and finishPartial keeps that issue's worktree alongside it so the
  retry's dispatch has somewhere to land. If the reused pane is gone by the time the retry
  runs (herdr restarted, a human closed it), dispatchViaHerdr falls back to one fresh
  dispatch rather than failing the round. A queued reuse nobody ever consumed (max-rounds,
  an unhandled error) is swept at the end of the run by main.mjs's new
  sprint.pendingHerdrReuses() loop, closing the pane and removing the worktree.