config: cleanup — remove old CloudLlmConfig / OrchestratorConfig / RouterConfig

[bug-ops]

Part of #2134 — Phase 6 (Cleanup).

Depends on: #2134 phases 1-4 all merged.

Spec: .local/specs/022-config-simplification/spec.md § Phase 6

Goal

Delete all legacy config types now that migration tooling is in place.

Structs to remove

• CloudLlmConfig
• OpenAiConfig
• GeminiConfig
• OllamaConfig (legacy form)
• CompatibleConfig (old form)
• OrchestratorConfig, OrchestratorProviderConfig
• RouterConfig
• Top-level LlmConfig fields: provider, base_url, model, embedding_model, vision_model (now on ProviderEntry)

Tasks

• Remove structs from crates/zeph-config/src/providers.rs
• Remove all re-exports referencing deleted types
• Update CHANGELOG.md with full list of removed fields
• Run /rust-release minor for version bump

Acceptance

• No CloudLlmConfig, OpenAiConfig, GeminiConfig, OrchestratorConfig, RouterConfig in codebase
• cargo +nightly fmt --check && cargo clippy --workspace --features full -- -D warnings clean
• All 6397 tests green
• CHANGELOG documents every removed field