fix(compaction): avoid repeating retained user instructions

[luoye520ww]

Problem

Part of #881. Context compaction could summarize content that the next model request also retained verbatim. That duplicated the latest user instruction; replayed compaction lifecycle events could also append duplicate side-conversation rows.

Root cause

The heuristic summary was built from the full history instead of only the folded head. The model-backed automatic and manual compaction paths could likewise pass retained-tail-derived skill pins through the continuation prompt. Their defensive fallback passed the full history when a compaction item had no sourceItemIds.

Scope

This PR makes compaction source selection strict and idempotent:

• Build heuristic summaries only from folded items.
• Feed model summarizers only the folded sourceItemIds for automatic and manual compaction.
• Derive pinned skill context only from those folded items, never from the retained tail.
• Fall back to the deterministic heuristic summary if folded source IDs are unavailable; never resend the full history.
• Update replayed side-conversation compaction lifecycle rows by stable itemId, preserving the original timestamp.

Non-goals

• This PR does not change configured model context windows or compaction thresholds.
• It does not infer a custom provider's context window. A model profile with no configured context window remains a separate diagnostic/configuration concern for reports of frequent compaction.

Safety

• No fuzzy or semantic deduplication is used.
• Textually identical user instructions from separate turns are retained: folded history is summarized and the newest copy stays verbatim in the tail.
• The latest user message is never removed from the retained tail.
• Missing folded-source metadata fails closed to the heuristic summary rather than risking tail resubmission.

Typecheck

Passed:

npm.cmd --prefix kun run typecheck
npm.cmd run typecheck

Tests

Passed:

npm.cmd --prefix kun test -- context-compactor.test.ts history-compaction-service.test.ts services/turn-service.test.ts loop/agent-loop.test.ts
npm.cmd exec -- vitest run src/renderer/src/store/chat-store-side-actions.test.ts
npm.cmd run lint
npm.cmd run build
git diff --check

The compaction tests exercise real HistoryCompactionService and TurnService flows with a local streaming model fixture for automatic and manual compaction; the renderer test replays lifecycle events through the side-conversation sink.

Review performed

• Functional correctness
• Architecture and source ownership
• Concurrency/lifecycle
• Data integrity
• Security and prompt-boundary handling
• Cross-platform build risk
• Test quality and PR scope

PR size

Production files: 5
Test files: 4
Total diff: +237 / -68

Issue

Part of #881