fix(OperationsCenter): stop Rule 8 retries on backend-capacity planner failures

[ProtocolWarden]

Summary

• board_worker now labels tasks blocked-reason: backend-capacity when a blocked task fails with the exact non-JSON + session limit or error backend signature.
• board_unblock Rule 8 (CLEAN_BLOCKED_RETRY) now excludes tasks carrying that label, so backend-capacity failures no longer get recycled into an immediate Blocked -> Backlog -> Ready for AI -> Blocked loop.
• Root cause: Rule 8 only excluded explicit dependency/signal/exit-code/policy-gate labels; a bare backend-capacity failure had no structured label, so it was treated as a generic clean retry and re-dispatched into the same failure repeatedly (closed-loop stagnation observed across the 2026-07-15 watchdog cycles).

Test plan

• pytest tests/unit/entrypoints/board_worker/test_outcomes_cov.py tests/unit/entrypoints/maintenance/test_board_unblock.py tests/unit/entrypoints/maintenance/test_board_unblock_cov.py -q → 202 passed
• pytest tests/unit/er000_phase0_golden/ -q → 15 passed