* feat: improve context compaction handoff summaries
Adapt PR #916 onto current main by replacing the old context summary marker
with a clearer handoff wrapper, updating the summarization prompt for
resume-oriented summaries, and preserving the current call_llm-based
compression path.
* fix: clearer error when docker backend is unavailable
* fix: preserve docker discovery in backend preflight
Follow up on salvaged PR #940 by reusing find_docker() during the new
availability check so non-PATH Docker Desktop installs still work. Add
a regression test covering the resolved executable path.
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Co-authored-by: aydnOktay <xaydinoktay@gmail.com>
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