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]>

docs/adr/0026-kimi-builtin.md

@@ -0,0 +1,60 @@
+# ADR-0026: Kimi (Moonshot AI) built-in provider
+
+**Status:** Accepted — 2026-07-18
+
+## Context
+
+Moonshot AI's Kimi models (Kimi K2, `moonshot-v1-*`, and the vision variants)
+are served over an OpenAI-compatible Chat Completions API at
+`https://api.moonshot.ai/v1` (`https://api.moonshot.cn/v1` for China),
+authenticated with a bearer key. mort wants Kimi as a first-class failover
+tier, so `kimi/kimi-k2-...` should parse, chain, and alias out of the box with
+a dedicated `KIMI_API_KEY` env var — the same ergonomics as `openai`,
+`anthropic`, and `google`.
+
+Two tensions:
+
+- The wire protocol is byte-for-byte OpenAI Chat Completions, so a hand-rolled
+  client would duplicate `provider/openai` for zero gain (ADR-0007 forbids it),
+  exactly as ADR-0015 found for llama-swap.
+- The README's current stance is that arbitrary OpenAI-compatible endpoints
+  (Groq, Together, …) are *consumer-registered*, not baked in. Blessing Kimi as
+  a built-in is a deliberate, narrow exception justified by the north star:
+  mort names Kimi directly in its tiers, and a built-in with `KIMI_API_KEY`
+  keeps mort's config free of boilerplate `openai.New(WithName/WithBaseURL)`
+  wiring.
+
+## Decision
+
+- **No new package.** The `kimi` built-in and `kimi://` DSN scheme both
+  construct `provider/openai` pointed at the Moonshot base URL — the chat path
+  inherits every openai feature/fix automatically (like llama-swap's chat).
+- The built-in reads its key through the registry's injected `envLookup`
+  (`KIMI_API_KEY` only — no `MOONSHOT_API_KEY` alias, per the project owner) so
+  it stays hermetically testable via `WithEnvLookup`.
+- **`WithAPIKey` is passed unconditionally, even when empty.** `openai.New`
+  defaults its key to `OPENAI_API_KEY`; without an explicit override an unset
+  `KIMI_API_KEY` would silently authenticate Kimi with the OpenAI key. Passing
+  the (possibly empty) lookup result severs that fallthrough.
+- New `openai.WithAPIKeyName("KIMI_API_KEY")` option customizes only the
+  synthetic-401 missing-key hint (default `OPENAI_API_KEY`), so a keyless kimi
+  call tells the operator the *right* variable to set.
+- The default endpoint is the international host (`kimiBaseURL`). The China
+  endpoint (or any other host) is reachable with a `kimi://` DSN, e.g.
+  `LLM_KCN=kimi://[email protected]/v1`. The `kimi://` scheme is an
+  OpenAI-compatible target labeled `kimi` with the same key-name hint; it is
+  intentionally near-identical to `openai://` — its value is a clear name in
+  specs and error reporting.
+
+## Consequences
+
+- `kimi/<model>` is first-class in Parse, chains, aliases, and health/failover
+  with no consumer wiring; model ids pass through verbatim (no catalog).
+- Chat, streaming, tools, and structured output ride the openai client. Image
+  *inputs* work at the client level but only the Moonshot vision models accept
+  them (matrix footnote ³).
+- `WithAPIKeyName` is a small, generally useful addition to `provider/openai`;
+  the default preserves existing behavior for every other openai-compat target.
+- Blessing one third-party endpoint as a built-in sets a precedent; future ones
+  should clear the same bar (a named consumer needs it in-config), not be added
+  reflexively — `RegisterProvider`/`LLM_*` remain the path for the rest.