CI: Fix full lane (coverage + full E2E)

[mjkatgithub]

Notes

• Full lane now runs on all MRs; fast lane only on feature-branch pushes.
• Fixed Synapse E2E on Linux CI: chown data dir after synapse generate so
  homeserver.yaml overrides are writable (EACCES on GitHub Actions).
• E2E runner: start-server-and-test third arg must be npm script name
  (see uncommitted package.json fix if not merged yet).
• Follow-up epics (Phase 4): Epic: Expand integration test suite #128 integration tests, Epic: Raise unit coverage toward critical-path targets #129 coverage ramp.
• Docs: N/A — CI/workflow and test-helper changes only.
• Branch protection: set required check "Full lane (coverage + full e2e)"
  for develop in GitHub Settings if not already configured.