Add read-only Action Scheduler diagnostics

[mehul0810]

Closes #181.

Summary

• adds a manual, cached, current-site Action Scheduler diagnostic with clean absent-table handling
• reports aggregate pending/running/complete/failed/canceled counts, oldest pending age, top hooks/groups, and action/log row indicators
• never reads action arguments, payloads, or log messages and never runs, cancels, deletes, or reschedules queue entries
• explicitly leaves failed-action trend unavailable because reliable derivation would require interpreting private log messages
• keeps future cleanup separate; no cleanup workflow is included in 1.8.0

Validation

• PHP lint and scoped PHPCS
• composer test — 143 tests, 471 assertions
• npm run test:unit — 40 tests
• npm run lint
• npm run build
• PHPStan full repository and changed-path follow-up
• git diff --check

Hosted WordPress smoke proof must confirm the absent-queue state, capability denial, clear-only-saved-snapshot behavior, and responsive desktop/mobile UI before merge.

[github-actions]

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