fix(agent): include the compaction boundary message in the summary

[cursor]

Bug and impact

When a session grows past max_context_messages, compaction deletes one persisted turn. That message is not included in the summary and is not kept in the tail, so a later load never sees it.

With the default cap of 40, a session of 50 messages (msg_0 through msg_49) drops msg_10. The existing test only checked that msg_11 survived, so the hole stayed green.

Root cause

compact_session_via_llm summarizes msg_count - max_ctx messages, then keeps max_ctx - 1 messages starting at msg_count - (max_ctx - 1). Those ranges do not meet. Index msg_count - max_ctx is in neither, and session_save writes the gapped history.

Fix

Summarize msg_count - max_ctx + 1 messages so the summary window ends where the kept tail starts. The stored session is still one summary plus max_ctx - 1 recent messages.

Validation

make test_agent failed on the new assertion (summarizer source must contain msg_10) before the fix and passes after it (test_agent: all tests passed).