Fix API key priority and compaction bugs

- getEnvApiKey: ANTHROPIC_OAUTH_TOKEN now takes precedence over ANTHROPIC_API_KEY
- findCutPoint: Stop scan-backwards loop at session header (was decrementing past it causing null preparation)
- generateSummary/generateTurnPrefixSummary: Throw on stopReason=error instead of returning empty string
- Test files: Fix API key priority order, use keepRecentTokens=1 for small test conversations

packages/coding-agent/examples/hooks/custom-compaction.ts

@@ -23,8 +23,15 @@ export default function (pi: HookAPI) {
 
 		ctx.ui.notify("Custom compaction hook triggered", "info");
 
-		const { messagesToSummarize, messagesToKeep, previousSummary, tokensBefore, resolveApiKey, entries, signal } =
-			event;
+		const {
+			messagesToSummarize,
+			messagesToKeep,
+			previousSummary,
+			tokensBefore,
+			resolveApiKey,
+			entries: _,
+			signal,
+		} = event;
 
 		// Use Gemini Flash for summarization (cheaper/faster than most conversation models)
 		const model = getModel("google", "gemini-2.5-flash");