Proposal: add a DebatePipeline (independent answers -> cross-examination -> judge synthesis)

[lss680455-create]

Summary

I propose adding a DebatePipeline to the new pipeline module: several peers answer the same question independently, then see each other's answers in one or more cross-examination rounds, then a single judge writes the final answer and records the disagreement that survives. It is the orchestration shape behind design reviews, review panels and contested analytical questions — and it is not expressible with the two existing pipelines (TeamPipeline keeps members isolated from each other; GoalPipeline is one executor plus one pass/fail verifier).

This is a proposal for #2479 ("if you have proposals for new pipeline types or designs, feel free to ... submit a proposal"). The full design — class and constructor, reply_stream pseudo-code, HITL / cancellation semantics, scope, and a 14-test matrix — is in issue #3828.

Why a pipeline rather than an example

Concurrency, HITL routing by reply_id, cancellation, round budget and context reset are exactly what the pipeline module exists to encapsulate. A single example would have to reimplement all of it, badly, and every user would repeat that work.

Design in one screen

• DebatePipeline(debaters: list[TeamMember], judge: Agent, rounds: int = 1, reset_debaters: bool = True)
• Round 0: every debater answers independently and concurrently; no peer content.
• Rounds 1..N: each debater receives its own previous answer plus the peers' previous answers, and must concede or restate its position.
• Judge: called once, receives the motion and the final positions; its streamed reply is the pipeline's answer.
• No vote counting in v1. "No consensus" is a first-class outcome the judge must name, not a failure.
• Reuses TeamMember, PipelineProtocol, and the HITL / cancellation machinery proven in TeamPipeline.

Open questions

1. Is pipeline/ the right home, or is this better as an example?
2. Reuse TeamMember, or introduce a dedicated Debater type?
3. Default rounds: 1 (one challenge round) or 2 (more dialectic, double the calls)?
4. Should the judge emit a structured verdict in v1, or stay free-form?

I will not open a PR until the direction is agreed.