Fix workflow foreground drain isolation

[chubes4]

Summary

• assign newly dispatched workflow runs durable run-specific Action Scheduler groups
• make foreground awaiters claim only the requested run, including strict HybridStore behavior
• preserve group and branch-store state across multiple fan-outs and guard resume actions by suspension generation
• retain the shared agents-api group for pre-upgrade in-flight runs without a mapping
• fall back to inline resume when durable resume enqueue fails

Verification

• composer test
• composer phpstan
• php tests/workflow-as-branch-smoke.php
• php tests/workflow-run-awaiter-smoke.php
• php tests/workflow-scoped-drain-smoke.php

Fixes #455