T44: pmat hunt in its own job chained from Coverage Nightly, so it reads the night's coverage #4875

Follow-up to #4872 (T44 producer chain).

Under the pick chain, qwen-story-daily and coverage-nightly both start when "Nightly pick" completes. The story job is capped at 30 min and coverage takes 60-70 min, so the pmat hunt's coverage lookup (gh run list -w coverage-nightly.yml --commit "$NIGHTLY_C") finds no completed run on most nights, and its coverage gaps read not_measured. That was the state before #4715.

Fix: move the pmat hunt into its own job or workflow, chained from Coverage Nightly finishing, measuring the same night's C. This changes what scripts/release/check_producers_chain.sh P2 accepts (a chain from a listed producer that itself chains from the pick), so it is a gate change and needs its own sign-off round and 2 non-author reviews.

Acceptance:

• the hunt reads coverage-nightly's coverage-json for the same NIGHTLY_C on a normal night (a run id in the log, not a warning)
• a failed or missing coverage run makes the hunt not_measured, never a pass
• the producer guard's self-test gains a row for the new chain plus a mutant that drops it

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