feat: add OMP (Oh My Pi) provider compatibility

Support OMP as a host alongside pi:
- Convert OMP's array-format system prompts to string via systemPromptToText
- Guard against OMP passing the literal env-var name as the API key value
- Add ~/.omp/agent/auth.json to default auth path lookup
- Fix --list-models output to check both stdout and stderr
- Add OMP compatibility smoke test with isolated temp HOME and mock server
- Deduplicate usage section in README, add OMP install and usage docs

package.json

@@ -28,7 +28,7 @@
     "LICENSE"
   ],
   "scripts": {
-    "test": "npm run typecheck && tsx tests/test-pure-functions.ts && tsx tests/test-models.ts && tsx tests/test-oauth.ts && tsx tests/test-abort.ts && tsx tests/test-stream.ts && node tests/test-pi-local.mjs",
+    "test": "npm run typecheck && tsx tests/test-pure-functions.ts && tsx tests/test-models.ts && tsx tests/test-oauth.ts && tsx tests/test-abort.ts && tsx tests/test-stream.ts && node tests/test-pi-local.mjs && node tests/test-omp-compat.mjs",
     "typecheck": "tsc --noEmit",
     "format:check": "prettier --check '**/*.{ts,mjs,json,md}'",
     "format": "prettier --write '**/*.{ts,mjs,json,md}'",