fix: normalize OpenAI/GitHub finish_reason to Flynn stopReason conventions

OpenAI-compatible providers return 'stop' and 'tool_calls' as finish_reason
values, but Flynn's agent loop expects Anthropic-style 'end_turn' and
'tool_use'. This caused the agent to exit the tool loop prematurely when
falling back to GitHub Copilot (due to Anthropic API quota exhaustion).

- openai.ts: Map 'stop' → 'end_turn', 'length' → 'max_tokens', tool_calls
  with actual tools → 'tool_use', tool_calls without tools → 'end_turn'
- github.ts: Handle edge case where finish_reason is 'tool_calls' but no
  tools were parsed
- agent.ts: Accept both 'tool_use' and 'tool_calls' as valid stop reasons
  (belt-and-suspenders), extract toolCalls to local variable for TS narrowing
- openai.test.ts: Update expectations to match new normalized values

src/models/github.ts

@@ -161,6 +161,9 @@ export class GitHubModelsClient implements ModelClient {
         stopReason = 'end_turn';
       } else if (reason === 'length') {
         stopReason = 'max_tokens';
+      } else if (reason === 'tool_calls') {
+        // Edge case: finish_reason says tool_calls but none were parsed
+        stopReason = 'end_turn';
       } else {
         stopReason = reason ?? 'end_turn';
       }