feat: add API key and custom provider configuration (#4)

* feat: add API key and custom provider configuration

Previously, model setup only offered OAuth login. This adds:

- API key configuration for 17 built-in providers (OpenAI, Anthropic,
  Google, Mistral, Groq, xAI, OpenRouter, etc.)
- Custom provider setup via models.json (for Ollama, vLLM, LM Studio,
  proxies, or any OpenAI/Anthropic/Google-compatible endpoint)
- Interactive prompts with smart defaults and auto-detection of models
- Verification flow that probes endpoints and provides actionable tips
- Doctor diagnostics for models.json path and missing apiKey warnings
- Dev environment fallback for running without dist/ build artifacts
- Unified auth flow: `feynman model login` now offers both API key
  and OAuth options (OAuth-only when a specific provider is given)

New files:
- src/model/models-json.ts: Read/write models.json with proper merging
- src/model/registry.ts: Centralized ModelRegistry creation with modelsJsonPath
- tests/models-json.test.ts: Unit tests for provider config upsert

* fix: harden runtime env and custom provider auth

---------

Co-authored-by: Advait Paliwal <advaitspaliwal@gmail.com>

tests/models-json.test.ts

@@ -0,0 +1,32 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { mkdtempSync, readFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+import { upsertProviderConfig } from "../src/model/models-json.js";
+
+test("upsertProviderConfig creates models.json and merges provider config", () => {
+	const dir = mkdtempSync(join(tmpdir(), "feynman-models-"));
+	const modelsPath = join(dir, "models.json");
+
+	const first = upsertProviderConfig(modelsPath, "custom", {
+		baseUrl: "http://localhost:11434/v1",
+		apiKey: "ollama",
+		api: "openai-completions",
+		authHeader: true,
+		models: [{ id: "llama3.1:8b" }],
+	});
+	assert.deepEqual(first, { ok: true });
+
+	const second = upsertProviderConfig(modelsPath, "custom", {
+		baseUrl: "http://localhost:9999/v1",
+	});
+	assert.deepEqual(second, { ok: true });
+
+	const parsed = JSON.parse(readFileSync(modelsPath, "utf8")) as any;
+	assert.equal(parsed.providers.custom.baseUrl, "http://localhost:9999/v1");
+	assert.equal(parsed.providers.custom.api, "openai-completions");
+	assert.equal(parsed.providers.custom.authHeader, true);
+	assert.deepEqual(parsed.providers.custom.models, [{ id: "llama3.1:8b" }]);
+});