Add model-judge provider

[pbdeuchler]

Summary

Adds model-judge slots for models.default and models.subagent. A model-judge slot is implemented as a synthetic ModelJudgeProvider, so the runtime still sees an ordinary Provider and the fanout/judging/default-model handoff stays inside the provider boundary.

Flow

1. Panel - the original request, including context and tool specs, is sent to every model in models.model_judge.panel concurrently.
2. Synthesis - the models.model_judge.synthesis model receives the labeled panel responses plus the injected rank_responses tool. It emits ranking telemetry, then writes synthesis guidance that judges the candidate responses rather than merging them directly.
3. Default - the synthesis text is appended as transient internal guidance to the request sent to models.model_judge.default. The default model's stream is returned to the caller and still owns all real tool execution.
4. Fallback - if the panel produces no usable responses, or synthesis fails, the provider falls back to the default model alone.

The synthesis guidance is not persisted to the session transcript and does not require a protocol-level message variant.

Config

models.default and models.subagent accept either the existing inline model table or the string "model_judge", which references one shared [models.model_judge] block:

[models]
default = "model_judge"
subagent = "model_judge"

[models.model_judge.default]
provider = "anthropic"
model = "claude-opus-4-8"

[models.model_judge.synthesis]
provider = "openai"
model = "gpt-5"

[[models.model_judge.panel]]
provider = "openai"
model = "gpt-5"

[[models.model_judge.panel]]
provider = "anthropic"
model = "claude-sonnet-4-6"

models.small remains a single concrete model. When it is unset and the default slot is "model_judge", it falls back to the model-judge default leaf instead of fanning out.

Telemetry

Panel responses, synthesis output, and ranking entries are emitted as structured tracing events on halter::model_judge.

Changes

• config: adds ModelSlot and ModelJudgeConfig with default, synthesis, and non-empty panel validation.
• providers: adds ModelJudgeProvider, panel collection, ranking telemetry, synthesis guidance, and fallback behavior.
• builder: resolves model-judge slots into synthetic providers while sharing provider-family clients across members.
• docs: updates the workspace README, halter-config README, and changelog for the model-judge config and flow.

Testing

• cargo test -q -p halter-config -p halter-providers -p halter --no-default-features
• cargo test -q --workspace --all-features