fix: OAuth token refresh failure returns undefined instead of throwing

When OAuth refresh fails during model discovery, getApiKey() now returns
undefined instead of throwing. This allows the app to start and fall back
to other providers, so the user can /login to re-authenticate.

fixes #498

.pi/prompts/is.md

@@ -0,0 +1,21 @@
+---
+description: Analyze GitHub issues (bugs or feature requests)
+---
+Analyze GitHub issue(s): $ARGUMENTS
+
+For each issue:
+
+1. Read the issue in full, including all comments and linked issues/PRs.
+
+2. **For bugs**:
+   - Ignore any root cause analysis in the issue (likely wrong)
+   - Read all related code files in full (no truncation)
+   - Trace the code path and identify the actual root cause
+   - Propose a fix
+
+3. **For feature requests**:
+   - Read all related code files in full (no truncation)
+   - Propose the most concise implementation approach
+   - List affected files and changes needed
+
+Do NOT implement unless explicitly asked. Analyze and propose only.