feat: Add SummarizingConversationManager

[jsamuel1]

Implements summarizing conversation manager that preserves context by summarizing older messages instead of simply trimming them.

Features

• Configurable summary ratio (0.1-0.8, default 0.3)
• Configurable recent message preservation (default 10)
• Two summarization paths: direct model call or dedicated agent
• Tool pair boundary detection to maintain conversation validity
• Async hook integration with AfterModelCallEvent

Testing

• 27 comprehensive unit tests covering all functionality
• All 841 tests pass with no type errors
• 94.28% statement coverage, 88.88% branch coverage

Closes #279

[github-actions]

Assessment: Request Changes

This PR introduces a well-implemented SummarizingConversationManager that closely follows the Python SDK design. The core functionality is solid with good test coverage.

Review Categories

• Unrelated Files: .kiro/settings/lsp.json should be removed from this PR
• Code Duplication: Tool pair boundary detection logic should be extracted to a shared utility with SlidingWindowConversationManager
• API Completeness: Consider adding session persistence methods (getState/restoreFromSession) for Python SDK parity
• API Review: This introduces a new public abstraction customers will frequently use - consider adding needs-api-review label per API Bar Raising guidelines

The implementation is well-structured and the tests are comprehensive. Good work aligning with the Python SDK design!

[guenhter]

@jsamuel1 Love to get this. Any update?

[jsamuel1]

@jsamuel1 Love to get this. Any update?

THanks for the prompt. I didn't see the comments above. Fixing now.

[jsamuel1]

All automated review feedback has been addressed in the latest push:

1. Unrelated file — .kiro/settings/lsp.json removed, .kiro/ added to .gitignore
2. Unused _summaryMessage field — Removed. Session persistence (getState/restoreFromSession) deferred to a follow-up issue covering all conversation managers.
3. Code duplication — Extracted shared findValidSplitPoint() utility in src/conversation-manager/utils.ts, used by both SlidingWindowConversationManager and SummarizingConversationManager. Added unit tests.
4. Summary role comment — Added explanatory comments for the user role design choice in both summary generation methods.

Re: API review label — happy to add needs-api-review if a maintainer thinks it's warranted.

[jsamuel1]

Re: session persistence — after merging upstream main, the new snapshot/session infrastructure (src/session/ + src/agent/snapshot.ts from #520 and #560) already handles this. takeSnapshot(agent) captures agent.messages which includes the post-summarization state, and loadSnapshot restores it. Since SummarizingConversationManager modifies agent.messages in-place, session persistence works out of the box without needing getState()/restoreFromSession() on the conversation manager itself.