feature: add policy-driven signal and model selection primitives

[Xunzhuo]

Problem

The router can compose boolean signal matches, but several policy inputs still require ad hoc integrations: caller-supplied metadata, reusable generic label classifiers, numeric score gates at decision leaves, and bounded prompt-driven choice among a decision's candidate models.

Proposed scope

• add bounded, untrusted request metadata and request-envelope facts as normal signals
• expose reusable local and external-LLM classifier labels with explicit score predicates and error policy
• add deterministic prompt-driven selection over declared modelRefs
• keep evaluation recipe-scoped and preserve diagnostics in replay
• provide canonical config validation, examples, documentation, and behavior-level coverage

Acceptance criteria

• existing configurations remain backward compatible
• new config fields round-trip through Router, CLI, and Dashboard contracts
• prompt/classifier output is schema-validated and cannot select undeclared labels or models
• local tests, smoke checks, and CI pass