feat(receipts): archive verification evidence before retention pruning

[solomonneas]

Closes #565

Summary

Verification retention kept the newest 50 run directories and deleted older receipt evidence, which conflicts with append-only audit storage. This adds an archival path that preserves receipt evidence before local pruning runs.

• archival path runs before local pruning
• receipt integrity metadata and schema version preserved through archival
• local retention limit stays configurable
• tests cover archive-before-prune ordering and integrity preservation

Provenance

Produced by brigade run --worker k3 --worktree, run 20260727-211319-f7ca4eae. That run is recorded as failed due to the isolation-guard misclassification filed as #597; the worker rebased its own worktree onto origin/main, which is why this patch applies to main cleanly.

Verification

brigade work verify run on tests/test_work_cmd_verification.py tests/test_config.py, receipt 20260727-215705-work-verify-dc6fa9: 137 passed, 1 failed.

The single failure is test_verify_reused_receipt_stamps_harness_session_from_outer_env_prefix, which fails identically on unpatched origin/main in a worktree (No module named brigade in a spawned subprocess, because a worktree lacks the editable install). It is pre-existing and environmental, not introduced here. CI runs against a proper install and should be the deciding gate.

[coderabbitai]

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