Add KNOWN_ISSUES.md listing reviewed bounty submissions

[pinebit]

Summary

Adds KNOWN_ISSUES.md, a public list of bug bounty submissions that have already been reviewed by the Obol Security Team for the OVM, OWR, and OTR contracts.

The document:

• Lists known findings grouped by category (reentrancy & accounting, access control & roles, other logic), with the rationale for each folded into the description — no internal triage verdicts or severity grades are exposed
• States that duplicate submissions are not eligible for a reward
• Links to the documented role risks and trust assumptions
• Directs new reports to security@obol.network