feat(providers): OpenAI-compatible provider types as runtime data (providers.yaml)

Replace the five copy-paste OpenAI-compatible provider structs (moonshot,
moonshot_code, deepseek, zai, lm_studio) with one DeclaredProvider engine
driven by a providers.yaml catalog loaded at boot from the cwd — edit and
restart, no rebuild. The YAML carries identity, endpoints, per-model JSON
field mapping, id-glob enrichment rules and the reasoning knob (effort /
thinking request kinds); capability_flags keep vision and future input
modalities declarative. Anthropic, Ollama, OpenAI and OpenRouter stay
native (different wire protocols or bespoke parsing) and register
alongside; colliding declared ids are skipped. The shipped catalog is
validated by a unit test.

crates/skald-core/Cargo.toml

@@ -42,6 +42,7 @@ uuid    = { version = "1", features = ["v4"] }
 reqwest = { version = "0.13.4", default-features = false, features = ["rustls-no-provider", "charset", "http2", "system-proxy", "json", "multipart"] }
 async-trait = "0.1"
 serde_json = "1"
+serde_yaml = "0.9"
 indexmap = { version = "2", features = ["serde"] }
 tracing            = "0.1"
 chrono             = { version = "0.4", default-features = false, features = ["clock", "std"] }