Add independent postcondition verification to AgentTrace episodes

[makslanies]

Problem

An executor can report success even when the intended effect never becomes observable. Examples include a push to a branch that no longer affects the default branch, or a deploy that exits successfully while production still serves the previous revision.

VOLY should distinguish action completion from outcome effectiveness in its multi-agent traces and episodes.

Source discussion: microsoft/autogen#7265 (comment)

Proposed direction

• Add an explicit postcondition/effectiveness result to AgentTrace / MultiAgentEpisode.
• Keep execution success and observed effect as separate semantics.
• Run the postcondition at the earliest point where the expected effect can be observed.
• Allow the verifier/observer to use a channel independent from the actor's execution channel.
• Persist verifier identity, observation timestamp, evidence/artifacts, verdict, and failure reason in the episode.
• Make the result available to Agentic Judge and role metrics.

Possible initial model:

PostconditionResult
- status: passed | failed | inconclusive | not_run
- observer_role
- observed_at
- assertion
- evidence_refs[]
- failure_reason
- channel

Acceptance criteria

• A successful executor result can coexist with a failed or inconclusive postcondition.
• Episode serialization preserves both action success and effectiveness.
• At least one deterministic postcondition implementation uses an observer path independent from the actor.
• Solver/Judge workflows can consume the postcondition result without write access.
• Role metrics can attribute prevention/detection value to the observer or judge.
• Tests cover success + failed effect, success + passed effect, and verifier-channel failure.
• Backend architecture and workflow/evaluation documentation are updated.

Non-goals

• Generic workflow engine.
• Self-play or training from postcondition data in the first iteration.
• Claiming an effect when the verifier itself fails; that state must remain inconclusive.