Add configurable OAuth storage backend and respect --models in model selector

- Add setOAuthStorage() and resetOAuthStorage() to pi-ai for custom storage backends
- Configure coding-agent to use its own configurable OAuth path via getOAuthPath()
- Model selector (/model command) now only shows models from --models scope when set
- Rewrite OAuth documentation in pi-ai README with examples

Fixes #255

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -1382,6 +1382,7 @@ export class InteractiveMode {
 				this.ui,
 				this.session.model,
 				this.settingsManager,
+				this.session.scopedModels,
 				async (model) => {
 					try {
 						await this.session.setModel(model);