Handle HybridStore missing group claims in scoped drain

[chubes4]

Summary

• Retry scoped Action Scheduler claims without the group only when HybridStore throws a missing-group InvalidArgumentException.
• Preserve hook scoping, so workflow branch/resume drains remain limited to the Agents API hooks.
• Keeps normal group-scoped claims unchanged on stores that support them.

Root cause

Live WP Cloud uses ActionScheduler_HybridStore. During foreground drain proof, branch actions were readable under the agents-api group, but stake_claim(..., hooks, agents-api) threw “The group agents-api does not exist”, so the scoped drain stopped with warning before processing pending branch actions.

Verification

• php -l src/Workflows/class-wp-agent-workflow-scoped-drain.php
• php tests/workflow-scoped-drain-smoke.php
• php tests/workflow-parallel-async-smoke.php

Related

• Follow-up for chubes4/wp-build#1028 / chubes4/wp-build#962.

AI assistance

• AI assistance: Yes
• Tool(s): GPT-5.5 via opencode
• Used for: Interpreted live WP Cloud Action Scheduler evidence, implemented the HybridStore fallback, and ran focused verification.