Mark turns interrupted when compression is cancelled

[uuzzrm]

What breaks

If a turn is cancelled while the context compressor is waiting on its LLM summary, Agent.chat() rejects before it reaches the existing cancellation handler. The new user message remains as a bare trailing user message, so a later request can treat the cancelled task as unfinished.

What changed

• Move the initial user-message append and maybeCompress() call inside the existing try block.
• Reuse the current [interrupted by user] history marker for cancellation during compression.
• Add a regression test that cancels a summary request and checks the resulting history.

Verification

• npm test — 55 passed, 1 skipped (the existing platform-specific SIGKILL test)
• git diff --check

The failure was reproduced locally with a cancellable LLM stub before the fix. This change was AI-assisted and reviewed against the current min branch.