feat(git): per-block conflict view with mine/theirs choices (#45)

Opening a conflicted file in the Changes panel now shows each conflict
block as its two sides, labelled from git's markers and tinted apart,
with Keep mine / Take theirs / Keep both per block (undoable, ^/v to
jump). Save and mark resolved writes the chosen text through the
whole-file editor's PUT /git/file (baseSha stale check) then runs
resolve done. The Merge conflicts group's buttons are text, and a
conflicted file is marked with a red ! in the Files tree.

Fix: ls-files --cached lists an unmerged path once per stage, so the
tree showed a conflicted file three times; lsFiles now dedupes.

server/__tests__/git-ops-merge.test.js

@@ -16,6 +16,7 @@ const { execFileSync } = require("node:child_process");
 
 const { runGitAction } = require("../lib/git-ops");
 const { gitFacts } = require("../lib/worktree");
+const { listTree, listPaths } = require("../lib/git-diff");
 
 const ROOT = fs.mkdtempSync(path.join(os.tmpdir(), "agenthub-merge-"));
 const REPO = path.join(ROOT, "repo");
@@ -87,6 +88,15 @@ describe("merge", () => {
   });
 });
 
+describe("listing files mid-merge", () => {
+  it("lists a conflicted file once, not once per merge stage", async () => {
+    await merge();
+    const root = await listTree(REPO, {});
+    assert.equal(root.entries.filter((e) => e.name === "a.txt").length, 1);
+    assert.deepEqual((await listPaths(REPO, { q: "a.txt" })).paths, ["a.txt"]);
+  });
+});
+
 describe("resolve", () => {
   it("takes our side or theirs, and only for a conflicted path", async () => {
     await merge();