F4 SLICE-1: proposalModel helpers — add/remove + fenced-diff body (#12)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/proposalModel.test.ts

@@ -9,6 +9,7 @@ import {
   type Proposal,
 } from "../src/model";
 import { CoauthorStore } from "../src/store";
+import { addProposal, proposalBody, removeProposal } from "../src/proposalModel";
 
 const agent = {
   kind: "agent" as const,
@@ -73,3 +74,34 @@ describe("CoauthorStore prune with proposals (spec §6.3)", () => {
     }
   });
 });
+
+describe("proposalModel helpers (spec §6.4)", () => {
+  const fp = { text: "old line one\nold line two", before: "", after: "", lineHint: 3 };
+
+  it("addProposal creates the anchor + pending record; removeProposal deletes by id", () => {
+    const a = emptyArtifact("docs/x.md");
+    const { proposalId, anchorId } = addProposal(a, fp, "new line", agent, {
+      turnId: "turn-9", instruction: "shorten",
+    });
+    expect(a.anchors[anchorId].fingerprint).toEqual(fp);
+    expect(a.proposals).toHaveLength(1);
+    expect(a.proposals[0].id).toBe(proposalId);
+    expect(a.proposals[0].replacement).toBe("new line");
+    expect(a.proposals[0].turnId).toBe("turn-9");
+    expect(a.proposals[0].instruction).toBe("shorten");
+    expect(removeProposal(a, proposalId)).toBe(true);
+    expect(a.proposals).toHaveLength(0);
+    expect(removeProposal(a, proposalId)).toBe(false);
+  });
+
+  it("proposalBody renders instruction + a fenced unified diff of old → new", () => {
+    const a = emptyArtifact("docs/x.md");
+    addProposal(a, fp, "new only line", agent, { instruction: "merge the lines" });
+    const body = proposalBody(fp.text, a.proposals[0]);
+    expect(body).toContain("**Claude proposes** — _merge the lines_");
+    expect(body).toContain("```diff");
+    expect(body).toContain("- old line one");
+    expect(body).toContain("- old line two");
+    expect(body).toContain("+ new only line");
+  });
+});