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/pi-runtime.test.ts

@@ -50,6 +50,7 @@ test("buildPiEnv wires Feynman paths into the Pi environment", () => {
 		assert.equal(env.FEYNMAN_NPM_PREFIX, "/home/.feynman/npm-global");
 		assert.equal(env.NPM_CONFIG_PREFIX, "/home/.feynman/npm-global");
 		assert.equal(env.npm_config_prefix, "/home/.feynman/npm-global");
+		assert.equal(env.PI_CODING_AGENT_DIR, "/home/.feynman/agent");
 		assert.ok(
 			env.PATH?.startsWith(
 				"/repo/feynman/node_modules/.bin:/repo/feynman/.feynman/npm/node_modules/.bin:/home/.feynman/npm-global/bin:",