fix(compaction): raise summarization wall-clock budget to 15 minutes

[leeseunguk]

Summary

DEFAULT_SUMMARIZATION_MAX_DURATION_MS was 120s. A 257k-token session summarizing on Grok 4.6 was still streaming when the cap fired, so compact was rejected and the session stayed above the threshold.

Keep a wall-clock budget, but size it for large-session summarization (900s). The 300s idle timeout still kills a silent connection.

Fixes #1068

Test plan

• bun test test/compaction/summarization-duration-constant.test.ts
• Existing summarization-wall-clock-budget.test.ts still expects the exported constant (now 900_000). Full file needs packages/ai installed; the new constant test does not.

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Summary by cubic

Raises the summarization wall-clock budget from 120s to 15 minutes so large-session compaction can complete. Previously, we aborted still-live summaries and left sessions above the compaction threshold; now large summaries can finish while the 300s idle timeout still ends silent connections.

• DEFAULT_SUMMARIZATION_MAX_DURATION_MS is now 900_000 ms in packages/coding-agent/src/core/compaction/stream-watchdog.ts and applies per attempt via retryAssistantCall.
• Idle timeout remains DEFAULT_SUMMARIZATION_IDLE_TIMEOUT_MS = 300_000 ms; silence still terminates hung streams.
• Tests: added summarization-duration-constant.test.ts; updated summarization-wall-clock-budget.test.ts to assert the new constant.

^{Written for commit 1fb922d. Summary will update on new commits.}