Why: tool-produced artifact_ref parts were appended without Process,
unlike the tool_call/tool_result parts of the same turn, so on final
reconcile the artifact chip stayed in the reply area instead of folding
into the process summary.
Evidence: llm.go now sets Process: true on PartArtifactRef next to its
tool parts; new TestLLMRunnerArtifactPartIsProcess drives a tool turn
that produces an artifact and asserts the part exists, is marked, and
carries the artifact ref. go test ./... all packages ok.
Impact: persisted assistant messages fold artifact references into the
collapsed process area, consistent with other tool-turn output.
Refs: code review of feature/chat-process-split-r1 (2026-09-30)
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