unify context overflow recovery

[deathbyknowledge]

Summary

• route provider-confirmed context overflows through the same Process-owned history policy used by preflight
• auto-compact and retry the same active model once instead of entering the main generation fallback chain
• document fail, no-prefix, and repeated-overflow behavior and cover the observed Workers AI/Kimi response

Validation

• cd gateway && npx tsc --noEmit
• focused Process overflow tests: 8 passed

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Highlight overflow compaction failures in chat

When auto-compaction fails after a provider-confirmed overflow, this emits a new system-message prefix, but the chat UI only recognizes Auto-compaction failed before model call: in web/src/app/features/chat/domain/transcript.ts:322-327. In that summary-generation/archive-failure path, the run finishes with status: "error" while both the live proc.changed row and refreshed history render as a neutral system message, hiding the failure from users; please update the web consumer alongside the new gateway format.

AGENTS.md reference: AGENTS.md:L165-L165

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