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Task failure isolation, agentic timeout fix, download module consolidation

Fixed
- Task failure isolation: untrusted-agent swap failures (VERIFY_PATHS download
  errors) are now per-task error outcomes instead of FatalEvaluationError —
  one broken task no longer aborts the whole job. Transient download errors
  retry with exponential backoff (2s→60s, 5 attempts); 404 retries once, then
  fails the task permanently. Judge config is validated in a preflight before
  dispatch, so misconfiguration fails in seconds instead of after a full
  agent run. (v1.3.1)
- Agentic per-task timeout raised 2h10m → 3h30m: the dispatch timeout now
  sits above the agent's graceful 3h timeout, so long tasks are cancelled
  server-side with metrics preserved instead of killed with a bare
  TimeoutError and an unreconciled vivarium run.
- Verify-stage infra errors (504s, connection errors) are excluded from the
  accuracy denominator instead of grading tasks as incorrect.

Changed
- Download module consolidation: engine moved to palace/download/engine.py;
  conversion_recipes.json is now the single source of truth for public
  benchmark recipes (public_datasets_info.json deleted). Legacy palace-cli
  and palace-run CLIs removed (entrypoints are now "removed" banner shims).
- omp is now the default vivarium agent harness.
- Public recipe catalogue trimmed: removed BABILong-32k/128k (superseded by
  the full-length BABILong), CURIE-protein, SimpleQA-Verified, VLSBench.