ci: remove obsolete merge MOC update workflow

[XuPeng-SH]

Summary

• Remove the obsolete post-merge MOC coverage-status workflow. The current coverage pipeline is self-contained and no longer creates the legacy MOC record this workflow expects.
• Stabilize three user-level-lock cleanup tests by asserting durable terminal state instead of transient asynchronous-worker ownership/progress, and always reset shared test state on early assertion exit.
• Add the read-only mo-pr-radar skill for deterministic PR review queues, conflict/CI/approval filters, request-changes freshness, and per-PR blocker explanations. Its review model uses each reviewer's latest opinionated review, so a later comment cannot erase an approval/change request and a later approval clears historical change requests.
• Strengthen mo-self-review with exact affected-test selection, adaptive per-test race budgets, deterministic flake checks, and one complete owning-package race run. Zero/under-resolution JSON test durations safely use the adaptive ceiling.

Validation

• Confirmed the removed workflow runs only after a merged PR closes, is not a required check on main or 4.2-dev, and has no repository/Mergify references.
• PR radar: 22 offline unit tests, Bash syntax checks, both skill validators, and live GitHub regressions: fix(replace): apply parent-side foreign key actions during REPLACE #25089 remains CHANGES_REQUESTED after a later comment; approved fix(logtail): propagate checkpoint iterator errors #26233 is absent from the stale-change queue.
• Adaptive race measurements from go test -json, with B=30s: release-cleanup T=0.07s, N=100; timed-out cleanup T=1.06s, N=28; successful-probe cleanup T=0.05s, N=100.
• Final focused race stress passed for all three exact tests (-count=100/28/100); matching non-race stress also passed.
• Full pkg/sql/plan/function owning package passed with -race -count=1.
• go build and go vet passed for pkg/sql/plan/function.
• git diff --check passed after merging the latest main.

Fixes #26262.
Related to #26253.

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