docs: add migration checklist for pure functional IR

packages/core/src/context/migration-plan.md

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+# The Ship of Theseus Migration Checklist
+
+- [ ] **Phase 1: Core Types (`ir/types.ts`)**
+  - [ ] Add `ConcreteNode` and `LogicalNode` types.
+  - [ ] Add `episodeId` (or generic `parentId`) to all `ConcreteNode`
+        interfaces.
+  - [ ] Add `replacesId` and `abstractsIds` pointers.
+  - [ ] Remove `variants` dictionary from `IrNode`.
+
+- [ ] **Phase 2: Processor Pipeline (`pipeline.ts`)**
+  - [ ] Delete `EpisodeEditor`.
+  - [ ] Define `ContextPatch`.
+  - [ ] Update `ContextProcessor` signature to accept `ProcessArgs` and return
+        `Promise<ContextPatch[]>`.
+
+- [ ] **Phase 3: The Reducer (`sidecar/orchestrator.ts`)**
+  - [ ] Update `executePipeline` and `executeTriggerSync` to act as a reducer.
+  - [ ] Map `ContextPatch` results onto the flat Ship array.
+
+- [ ] **Phase 4: Pristine Graph & Mapping (`contextManager.ts` & `ir/toIr.ts`)**
+  - [ ] Update `toIr` to produce a flat list of `ConcreteNode`s and a tree of
+        `LogicalNode`s.
+  - [ ] Make `ContextManager` track the Pristine Graph and instantiate the flat
+        Ship.
+  - [ ] Commit patches to the Pristine Graph history.
+
+- [ ] **Phase 5: The Walker (`ir/projector.ts`)**
+  - [ ] Update projection to simply walk the flat `ReadonlyArray<ConcreteNode>`.
+  - [ ] Skip nodes whose IDs are in a "skipped" set (based on `abstractsIds`).
+
+- [ ] **Phase 6: Refactoring Processors**
+  - [ ] `ToolMaskingProcessor`
+  - [ ] `SemanticCompressionProcessor`
+  - [ ] `BlobDegradationProcessor`
+  - [ ] `EmergencyTruncationProcessor`
+  - [ ] `HistorySquashingProcessor`
+  - [ ] `StateSnapshotProcessor`