OPS-125 M-DEADSEAT: add maintainer-registry check as a third, separate alert line

pmmaster ruling 2026-09-12 (OPS-125 follow-up): a dead wi-assignee seat can also be a live maintainer-registry entry (coder-basquetwi-cc: unowned in wi, dormant by row/agent signal, but named as owner of the wi CLI across three active maintainer-agents.sqlite rows - a registry defect, not a row/dormancy defect; row/commit analysis cannot surface it). Add a third, cheap signal: SELECT against /var/lib/llmmsg-srv/maintainer-agents.sqlite maintainers table for any row naming this dead seat in maintainerAgentName or maintainerAgentBackup. One live-SSOT query, no new infra, no mapping to go stale. Report as its own line in M-DEADSEAT's alert, never folded into the dormant/live label - a dead seat with a registry row is an alarm regardless of how quiet its project is. Deferred behind KPI-58 (done)/MSG-207/KPI-31.