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]>

README.md

@@ -121,6 +121,7 @@ Chains are health-tracked per target:
 | Provider | Spec name | Key env var | Default endpoint |
 |----------|-----------|-------------|------------------|
 | OpenAI (+compatible) | `openai` | `OPENAI_API_KEY` | https://api.openai.com/v1 |
+| Kimi (Moonshot AI) | `kimi` | `KIMI_API_KEY` | https://api.moonshot.ai/v1 |
 | Anthropic (+compatible) | `anthropic` | `ANTHROPIC_API_KEY` | https://api.anthropic.com |
 | Google (Gemini) | `google` | `GOOGLE_API_KEY` / `GEMINI_API_KEY` | Gemini API (official SDK) |
 | Ollama Cloud | `ollama-cloud` | `OLLAMA_API_KEY` | https://ollama.com |
@@ -128,6 +129,11 @@ Chains are health-tracked per target:
 | foreman | `foreman` | — (token via DSN) | requires an LLM_* DSN or `ollama.Foreman(url, token)` |
 | llama-swap | `llama-swap` | — (token via DSN) | requires an LLM_* DSN or `llamaswap.New(...)` |
 
+Kimi is Moonshot AI's OpenAI-compatible Chat Completions endpoint, so it reuses
+the openai client (like llama-swap). The `kimi` built-in defaults to the
+international endpoint; reach the China endpoint (or any other host) with a
+`kimi://` DSN, e.g. `LLM_KCN=kimi://[email protected]/v1`.
+
 OpenAI-compatible / Anthropic-compatible endpoints: construct the provider
 with a name and base URL and register it —
 
@@ -159,7 +165,7 @@ m, _ := reg.Parse("m5/qwen3:30b,m1/qwen3:30b,thinking")
 ```
 
 DSN format: `scheme://[token@]host[/path]`, scheme ∈ `foreman`, `ollama`,
-`ollama-cloud`, `openai`, `anthropic`, `google`/`gemini`, `llama-swap`,
+`ollama-cloud`, `openai`, `kimi`, `anthropic`, `google`/`gemini`, `llama-swap`,
 `llama-swaps`, or any scheme you add with `RegisterScheme`. The token is the
 credential (bearer token / API key); the base URL is always `https://host[/path]`
 — except `llama-swap`, which builds `http://host[:port]` since it's local-first
@@ -400,6 +406,7 @@ to build one.
 | Provider | Resolve/Parse | Chat | Streaming | Tools | Structured | Images | Env DSN |
 |----------------------|:---:|:---:|:---:|:---:|:---:|:---:|:---:|
 | OpenAI (+compatible) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
+| Kimi (Moonshot AI) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅³ | ✅ |
 | Anthropic (+compat) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
 | Google (Gemini) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
 | Ollama Cloud | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
@@ -420,6 +427,10 @@ transcription** (`audio`), **video generation** (`videogen`) — separate
 axes, not shown above — plus a `Health`
 probe and management methods on `*llamaswap.Provider`.
 
+³ Kimi reuses the openai client, so image *inputs* are supported at the client
+level; whether a call succeeds depends on the Moonshot model — only the vision
+variants (e.g. `moonshot-v1-8k-vision-preview`) accept images.
+
 Notes: Ollama has no native tool_choice — `"none"` drops the tools;
 `"required"`/named choices are best-effort ignored there. Ollama Cloud
 ignores the `format` field (verified live), so the provider also states