workspace: reject mixed reset mode

Return exit 129 for git reset --mixed so the CLI matches its
documented unsupported-mode contract instead of silently treating
the command like a bare reset.

Cover hard reset from detached HEAD and reset that path by
checking out the resolved oid directly, avoiding a write through
the symbolic HEAD name.

Author: aron-cf

packages/workspace/src/git/cli.test.ts

@@ -1891,6 +1891,14 @@ describe("runGitCli — reset argv parsing", () => {
     expect(res.exitCode).toBe(129);
     expect(res.stderr).toContain("--soft is not supported");
   });
+
+  it("--mixed is rejected as unsupported", async () => {
+    const { client, calls } = fakeClient();
+    const res = await runGitCli(client, { argv: ["reset", "--mixed", "HEAD"] });
+    expect(res.exitCode).toBe(129);
+    expect(res.stderr).toContain("--mixed is not supported");
+    expect(calls.reset).toEqual([]);
+  });
 });
 
 describe("runGitCli — clean argv parsing", () => {

packages/workspace/src/git/cli.ts

@@ -2005,6 +2005,9 @@ async function runReset(
   if (parsed.flags.soft === true) {
     return { stdout: "", stderr: "git reset: --soft is not supported\n", exitCode: 129 };
   }
+  if (parsed.flags.mixed === true) {
+    return { stdout: "", stderr: "git reset: --mixed is not supported\n", exitCode: 129 };
+  }
   const sep = args.indexOf("--");
   const positional =
     sep === -1 ? parsed.positional : args.slice(0, sep).filter((a) => !a.startsWith("-"));

packages/workspace/src/git/worktree.test.ts

@@ -114,6 +114,22 @@ describe("resetWith", () => {
     expect(await statusOf("a.txt")).toEqual([1, 1, 1]);
   });
 
+  it("hard reset works from detached HEAD", async () => {
+    await init();
+    const first = await commit("a.txt", "v1\n", "first");
+    const second = await commit("a.txt", "v2\n", "second");
+    await git.checkout({ fs: memfs, dir: DIR, ref: second });
+    expect(await git.currentBranch({ fs: memfs, dir: DIR })).toBeUndefined();
+    await memfs.promises.writeFile(`${DIR}/a.txt`, "dirty\n");
+
+    await resetWith({ git: resetClient, fs: memfs, dir: DIR, hard: true, ref: first });
+
+    expect(await git.resolveRef({ fs: memfs, dir: DIR, ref: "HEAD" })).toBe(first);
+    expect(await git.currentBranch({ fs: memfs, dir: DIR })).toBeUndefined();
+    expect(await memfs.promises.readFile(`${DIR}/a.txt`, "utf8")).toBe("v1\n");
+    expect(await statusOf("a.txt")).toEqual([1, 1, 1]);
+  });
+
   it("hard reset throws NotARepositoryError outside a repo", async () => {
     await memfs.promises.mkdir("/loose", { recursive: true });
     await expect(

packages/workspace/src/git/worktree.ts

@@ -176,8 +176,7 @@ async function hardReset(opts: ResetWithDeps, dir: string, ref: string): Promise
     await opts.git.checkout({ fs: opts.fs, dir, ref: branch, force: true, cache: opts.cache });
     return;
   }
-  await opts.git.writeRef({ fs: opts.fs, dir, ref: "HEAD", value: oid, force: true });
-  await opts.git.checkout({ fs: opts.fs, dir, ref: "HEAD", force: true, cache: opts.cache });
+  await opts.git.checkout({ fs: opts.fs, dir, ref: oid, force: true, cache: opts.cache });
 }
 
 async function stagedPaths(opts: ResetWithDeps, dir: string): Promise<string[]> {