refactor: rename llm worker crate to engine

crates/llm-engine/README.md

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+# llm-engine
+
+## Role
+
+`llm-engine` owns provider-independent model turn orchestration over committed history, tools, callbacks, retries, continuation, pruning, and compaction boundaries.
+
+## Boundaries
+
+Owns:
+
+- Engine history mutation and append contracts
+- tool-call loop semantics
+- pre-stream retry and stream-started continuation policy
+- pruning/compaction coordination from the Engine perspective
+- provider-neutral events/callbacks/interceptors
+
+Does not own:
+
+- Pod names, sockets, process lifecycle, or scope delegation (`pod`)
+- product CLI shape (`yoi`)
+- provider catalog and secret resolution (`provider`, `secrets`)
+- durable Pod current state (`pod-store`)
+
+## Design notes
+
+The Engine is where turn lifecycle belongs because it sees history, in-flight usage, partial output, and tool-call state. It should not receive context-only volatile facts; model-affecting inputs must first be appended to history.
+
+## See also
+
+- [`../../docs/design/context-history.md`](../../docs/design/context-history.md)
+- [`../../docs/design/compaction.md`](../../docs/design/compaction.md)
+- [`../../docs/design/provider-model-boundary.md`](../../docs/design/provider-model-boundary.md)