docs: reframe README around agent supervision

[c-h-]

Summary

• update README positioning around agent supervision and universal control plane
• remove speculative headless-coding roadmap copy in favor of current operator value
• emphasize day-to-day workflows like launch, monitor, resume, interrupt, and compare across adapters

Notes

• local pre-push checks hit an existing flaky test in src/launch-orchestrator.test.ts (generateGroupId uniqueness assertion); this change is README-only

Testing

• not applicable (README only)

Co-Authored-By: Charlie Hulcher charlie.hulcher@gmail.com