qol-sweep: README polish + plugin-ecosystem cross-reference

[intel352]

Follow-up from the 2026-05-19 workflow QoL sweep. modular is the foundation lib for workflow + ratchet + cloud — needs its own OSS-readiness pass.

Scope

• README polish (badges, 60-sec quickstart, link to workflow + ratchet downstream consumers)
• CONTRIBUTING.md GOWORK=off pattern alignment with workflow#721
• docs/EXAMPLES.md auto-generated index of example configs (if applicable)
• Module-sub-package versioning matrix table (auth/cache/chimux/eventbus/etc.)

References

• Umbrella design: docs: workflow main README polish + examples index + plugin templates workflow#714
• Template: GoCodeAlone/workflow/docs/templates/
• Sweep memory: project_qol_sweep_2026_05_19_shipped.md