refactor: share LLM retry classification

[kermanx]

Related Issue

No linked issue. This PR follows up on consolidating duplicate retry classification used by regular turns and compaction.

Problem

Regular turn generation and compaction used equivalent but separate checks for retryable LLM failures. Keeping those checks in separate places makes it easier for retry behavior to drift as provider error handling evolves.

What changed

• Added a shared retry classification helper in the LLM abstraction layer for transient generation failures.
• Updated compaction retry handling and the Kosong-backed loop adapter to use the shared helper.
• Moved Kosong LLM construction behind the agent so turn flow no longer duplicates provider and completion budget wiring.
• Preserved the loop-facing finish reason contract by translating missing provider diagnostics to omitted optional fields.
• Added focused coverage for retryable and non-retryable provider errors.
• Added a patch changeset for the affected packages and CLI bundle.

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.

Verification

• pnpm --filter @moonshot-ai/agent-core typecheck
• pnpm --filter @moonshot-ai/kosong typecheck
• pnpm exec vitest run packages/kosong/test/errors.test.ts packages/agent-core/test/agent/turn.test.ts packages/agent-core/test/agent/compaction.test.ts packages/agent-core/test/agent/kosong-llm.test.ts

[changeset-bot]

🦋 Changeset detected

Latest commit: 0119c64

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

This PR includes changesets to release 3 packages

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

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