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/pr.md

@@ -0,0 +1,35 @@
+---
+description: Review PRs from URLs with structured issue and code analysis
+---
+You are given one or more GitHub PR URLs: $@
+
+For each PR URL, do the following in order:
+1. Read the PR page in full. Include description, all comments, all commits, and all changed files.
+2. Identify any linked issues referenced in the PR body, comments, commit messages, or cross links. Read each issue in full, including all comments.
+3. Analyze the PR diff. Read all relevant code files in full with no truncation. Include related code paths that are not in the diff but are required to validate behavior.
+4. Check for a changelog entry in the relevant `packages/*/CHANGELOG.md` files. Report whether an entry exists. If missing, state that a changelog entry is required before merge and that you will add it if the user decides to merge. Follow the changelog format rules in AGENTS.md.
+5. Provide a structured review with these sections:
+   - Good: solid choices or improvements
+   - Bad: concrete issues, regressions, missing tests, or risks
+   - Ugly: subtle or high impact problems
+6. Add Questions or Assumptions if anything is unclear.
+7. Add Change summary and Tests.
+
+Output format per PR:
+PR: <url>
+Changelog:
+- ...
+Good:
+- ...
+Bad:
+- ...
+Ugly:
+- ...
+Questions or Assumptions:
+- ...
+Change summary:
+- ...
+Tests:
+- ...
+
+If no issues are found, say so under Bad and Ugly.