Refactor Coder.hs to use the same agent core as Omni/Agent.hs:
Currently Coder has its own agent loop - unify with the standard one.
Files: Omni/Agent/Subagent/Coder.hs, possibly Omni/Agent.hs to expose internals
Merged Coder refactor to use Op-based agent core (Sequential interpreter + OpAgent) with same guardrails.
Refactored Omni/Agent/Subagent/Coder.hs to run via Op-based agent core (Seq.runSequential + OpAgent.runAgent) with budget/iteration limits and trace-based callbacks. Kept init/verify/commit/recovery phases intact and preserved cost tracking. bild Omni/Agent/Subagent/Coder.hs timed out after 10 minutes.