Deprecate old Omni/Agent/Coder.hs module

There are two Coder modules causing confusion:

• Omni/Agent/Coder.hs - Old task-based coder (appears unused)
• Omni/Agent/Subagent/Coder.hs - New hardened coder with init/verify/commit phases

The subagent system uses Subagent/Coder.hs. Either delete the old one or add clear deprecation notice. Verify nothing actually imports it first.