config: add ProviderEntry and RoutingStrategy types

[bug-ops]

Part of #2134 — Phase 1.

Spec: .local/specs/022-config-simplification/spec.md § Phase 1

Goal

Add new config types to zeph-config without touching existing code. No behavior change.

Tasks

• Add ProviderEntry struct to crates/zeph-config/src/providers.rs
• Add RoutingStrategy enum (None, Ema, Thompson, Cascade, Task)
• Add new LlmConfig definition with providers: Vec<ProviderEntry> alongside old one
• Re-export new types from crates/zeph-config/src/lib.rs
• Unit tests: ProviderEntry and RoutingStrategy have correct Default impls

Acceptance

• New types compile with --features full
• All 6397 tests green
• Old config code untouched

[bug-ops]

Implemented in feat-issue-2134-config-unified-providers — ProviderEntry, LlmRoutingStrategy, CandleInlineConfig, validate_pool() added. All 6397 tests pass.