fix(journal): re-init reactivity and drop var del

Make the reactivity engine re-initialize when the loaded declarations
change instead of only at import. ensureReactivity() reads the live
completions signal, compares a signature, and rebuilds from the store
(with exact bases) only when needed; dispatchCommand calls it before each
dispatch. Remove the unused DispatchContext.declarations/tagModifiers
fields, which were mount-time snapshots and could not be trusted.

Build initReactivity's maps into locals and commit only after the
circular check passes, so a failed re-init leaves the previous engine
intact; clear inFlight on commit.

Drop the unreachable var "del" action from the schema, render, and
reducer.

src/cli/completions/variable-system.test.ts

@@ -517,6 +517,33 @@ describe("var-reactivity", () => {
         }),
       ).not.toThrow();
     });
+
+    test("keeps the previous configuration when a re-init fails", () => {
+      const warn = jest.spyOn(console, "warn").mockImplementation(() => {});
+      initReactivity({
+        declarations: [{ key: "$hp", expression: "10 + 5" }],
+        tagModifiers: [],
+      });
+      expect(computeInitialValues(store()).find((r) => r.key === "$hp")?.value).toBe(
+        "15",
+      );
+
+      expect(() =>
+        initReactivity({
+          declarations: [
+            { key: "$a", expression: "$b + 1" },
+            { key: "$b", expression: "$a + 1" },
+          ],
+          tagModifiers: [],
+        }),
+      ).toThrow();
+
+      // The failed re-init did not clobber the working configuration.
+      const results = computeInitialValues(store());
+      expect(results.find((r) => r.key === "$hp")?.value).toBe("15");
+      expect(results.find((r) => r.key === "$a")).toBeUndefined();
+      warn.mockRestore();
+    });
   });
 
   describe("computeInitialValues", () => {