fix(agent): keep history that does not fit the summary

[cursor]

Bug and impact

Session compaction deleted history that the summarizer never saw. After a long chat, a large earlier message (for example a pasted log) followed by a short one in the summarize window was removed from the session, and the summary was built only from the short line. That history is gone from sessions.messages.

Root cause

compact_session_via_llm walks the oldest messages into a 64 KiB summary source, but it still drops every message before the kept tail. When a line does not fit, snprintf leaves a partial line and the used length does not advance, so the next shorter line overwrites it. The long message is neither summarized nor kept.

Fix

Append a line only when the whole line fits, and roll back a partial write. If the summary source stops early, keep that message and everything after it. If nothing fits, leave the session unchanged.

Validation

make test_agent passes. The new regression builds a session whose second summarize-window message is 65 KiB and checks that BIG_MARKER_END!! remains after compaction while the short prefix is still summarized. Before the fix that test failed with compacted session dropped the message that did not fit the summary.

  

[adriannoes]

Superseded by #121: consolidated with #102 into the operator-authored branch fix/agent-summary-compaction (same fixes, re-validated with make test_agent, CI=true make test, make static). Closing in favor of the consolidation.