fix(agent): skip compression when latest summary leaves no messages to summarize

[ms-alan]

Closes #59496

When a persisted handoff summary sits in the protected head after resume, the compression window recomputes turns_to_summarize as an empty slice. Previously Hermes would call _generate_summary([]), which triggers a noisy 'Context compression aborted' warning with no actual data loss.

This adds an early-return guard immediately after the slice is recomputed, mirroring the earlier compress_start >= compress_end no-op path and avoiding an unnecessary context engine request.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #59526 — both add the identical if not turns_to_summarize: early-return guard at the same site in agent/context_compressor.py (verified from the diffs), and both close #59496. #59526 is the earlier (opened 09:31 vs this PR's 16:07) and still-open canonical PR; #59526 also carries a regression test. Marking this later PR as a duplicate for tracking — maintainer to pick.

[teknium1]

Thanks for the focused compression no-op guard. Current main still recomputes the post-handoff slice at agent/context_compressor.py:3105-3108 and calls _generate_summary(...) unconditionally at agent/context_compressor.py:3139-3141, so the proposed guard addresses a real path.

Problems

• This branch has no regression test. The existing protected-head continuity case in tests/agent/test_context_compressor_summary_continuity.py:72-87 verifies a non-empty update path, not the empty post-handoff window.

Suggested changes

• Add a test that fixes the tail cut directly after the latest handoff summary and asserts no summary call occurs, the original messages are returned, and ineffective-compression accounting is updated.

This is an automated hermes-sweeper review.

[teknium1]

Thanks @ms-alan — you independently identified the empty post-handoff window bug (and the correct early-return site). The fix landed via salvage PR #70251, built on #59526 which was the 7-hours-earlier duplicate carrying a regression test — per our earliest-submitter convention that PR was the vehicle, but your identification is credited in the salvage PR body. Closing as landed on main.