feat: kimi (Moonshot AI) built-in provider (ADR-0026)

Add a first-class `kimi` provider and `kimi://` DSN scheme for Moonshot AI's
OpenAI-compatible Chat Completions API. Both reuse provider/openai (no new
client, mirroring llama-swap's chat path). Default endpoint is the
international host; the China endpoint is reachable via a kimi:// LLM_* DSN.

- Credential is KIMI_API_KEY, read through the registry's injected envLookup
  so it stays hermetically testable. WithAPIKey is passed unconditionally so
  an unset KIMI_API_KEY can never fall through to the openai client's
  OPENAI_API_KEY default.
- New openai.WithAPIKeyName option customizes the missing-key error hint
  (default OPENAI_API_KEY); kimi names KIMI_API_KEY.
- Hermetic tests: built-in base URL + bearer, missing-key hint names
  KIMI_API_KEY with no OPENAI fallthrough and no network hit, kimi:// scheme
  round-trips against the China host.
- Docs in sync: README built-in table + DSN scheme list + support matrix,
  .env.example, env.go DSN doc, ADR-0026 (+ index, backfilling 0024/0025),
  progress.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

builtin.go

@@ -13,7 +13,11 @@ import (
 
 // Built-in provider names.
 const (
-	ProviderOpenAI      = "openai"
+	ProviderOpenAI = "openai"
+	// ProviderKimi is Moonshot AI's Kimi models over their OpenAI-compatible
+	// Chat Completions endpoint. Reuses the openai client (like llama-swap);
+	// keyed by KIMI_API_KEY, default base URL kimiBaseURL.
+	ProviderKimi        = "kimi"
 	ProviderAnthropic   = "anthropic"
 	ProviderGoogle      = "google"
 	ProviderOllama      = "ollama"
@@ -28,6 +32,10 @@ const (
 	ProviderLlamaSwapTLS = "llama-swaps"
 )
 
+// kimiBaseURL is Moonshot AI's international OpenAI-compatible endpoint. The
+// China endpoint (api.moonshot.cn/v1) is reachable via a kimi:// LLM_* DSN.
+const kimiBaseURL = "https://api.moonshot.ai/v1"
+
 // registerBuiltins installs the built-in providers and env-DSN scheme
 // factories into a fresh registry. httpClient, when non-nil, is used by
 // every provider and factory the registry itself constructs.
@@ -74,6 +82,29 @@ func registerBuiltins(r *Registry, httpClient *http.Client) {
 		)...), nil
 	}
 
+	// Kimi (Moonshot AI): OpenAI-compatible Chat Completions, so it reuses the
+	// openai client (like llama-swap). Defaults to Moonshot's international
+	// endpoint and the KIMI_API_KEY credential. WithAPIKey is passed
+	// unconditionally — even empty — so an unset KIMI_API_KEY can never fall
+	// through to the openai client's OPENAI_API_KEY default; WithAPIKeyName
+	// makes the missing-key error name KIMI_API_KEY.
+	r.providers[ProviderKimi] = openai.New(openaiOpts(
+		openai.WithName(ProviderKimi),
+		openai.WithBaseURL(kimiBaseURL),
+		openai.WithAPIKey(r.envLookup("KIMI_API_KEY")),
+		openai.WithAPIKeyName("KIMI_API_KEY"),
+	)...)
+	// kimi:// DSN scheme: an OpenAI-compatible target labeled kimi, base URL
+	// from the DSN host (e.g. kimi://[email protected]/v1 for China).
+	r.schemes[ProviderKimi] = func(name string, dsn DSN) (llm.Provider, error) {
+		return openai.New(openaiOpts(
+			openai.WithName(name),
+			openai.WithBaseURL(dsn.BaseURL()),
+			openai.WithAPIKey(dsn.Token),
+			openai.WithAPIKeyName("KIMI_API_KEY"),
+		)...), nil
+	}
+
 	// llama-swap: OpenAI-compatible chat + image generation + management
 	// endpoints over a model-swapping proxy. Chat reuses the openai client
 	// (provider/llamaswap delegates). Two schemes: "llama-swap" builds an