fix: back off compaction overflow retries by token budget

[kermanx]

Related Issue

No linked issue. This follows up on #207 after checking the exported session and finding that overflow compaction retries can spend the retry budget on tiny trailing messages.

Problem

When a compaction summary request exceeds the model context window, the retry path used to move the compacted prefix back to the previous safe split only. In the reported session, the tail contained repeated tiny continue turns and empty assistant placeholders, so each retry reduced the provider input by only a few tokens and never reached the large tool-result messages before the retry budget was exhausted.

What changed

• Add minOverflowReductionRatio to CompactionConfig, defaulting to 0.05.
• Change overflow compaction backoff so each retry removes at least that fraction of the model context window in estimated tokens, while still choosing only safe split points.
• Add a regression test that covers repeated tiny trailing messages.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

Validation

• Replayed the exported session wire in memory up to line 1549. Old first retry would remove 6 estimated tokens; the new first retry removes 15,183 estimated tokens for a 262,144-token context window, above the 13,108-token 5% floor.
• pnpm vitest run packages/agent-core/test/agent/compaction.test.ts --testNamePattern "backs off overflow compaction"
• pnpm vitest run packages/agent-core/test/agent/compaction.test.ts
• pnpm --filter @moonshot-ai/agent-core run typecheck
• pnpm exec oxlint --type-aware packages/agent-core/src/agent/compaction/strategy.ts packages/agent-core/test/agent/compaction.test.ts
• git diff --check

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🦋 Changeset detected

Latest commit: d663d6c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@moonshot-ai/agent-core	Patch
@moonshot-ai/kimi-code	Patch

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commit: d663d6c

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