feat: add coder exp sync list command (#26443)

Add a new subcommand to list all registered sync units and their current
statuses. This provides a quick overview of the dependency coordination
state in a workspace without needing to query each unit individually.

The command supports both table (default) and JSON output formats.

```
$ coder exp sync list
UNIT    STATUS     READY
unit-a  started    true
unit-b  completed  true
unit-c  pending    false

$ coder exp sync list --output json
[
  {
    "unit_name": "my-unit",
    "status": "started",
    "is_ready": true
  }
]
```

When no units are registered, the command prints `No units registered`.

<details><summary>Changes across layers</summary>

- `agent/unit`: add `Manager.ListUnits()` method
- `agent/agentsocket/proto`: add `SyncList` RPC, bump API to v1.2
- `agent/agentsocket`: add service and client implementations
- `cli`: add `sync_list.go` command, register in `sync.go`
- Tests: three golden-file test cases (empty list, multiple units, JSON)

</details>

> Generated by Coder Agents on behalf of @SasSwart

---------

Co-authored-by: Cian Johnston <cian@coder.com>

cli/sync_test.go

@@ -409,4 +409,81 @@ func TestSyncCommands_Golden(t *testing.T) {
 
 		clitest.TestGoldenFile(t, "TestSyncCommands_Golden/status_json_format", outBuf.Bytes(), nil)
 	})
+
+	t.Run("list_no_units", func(t *testing.T) {
+		t.Parallel()
+		path, cleanup := setupSocketServer(t)
+		defer cleanup()
+
+		ctx := testutil.Context(t, testutil.WaitShort)
+
+		var outBuf bytes.Buffer
+		inv, _ := clitest.New(t, "exp", "sync", "list", "--socket-path", path)
+		inv.Stdout = &outBuf
+		inv.Stderr = &outBuf
+
+		err := inv.WithContext(ctx).Run()
+		require.NoError(t, err)
+
+		clitest.TestGoldenFile(t, "TestSyncCommands_Golden/list_no_units", outBuf.Bytes(), nil)
+	})
+
+	t.Run("list_with_units", func(t *testing.T) {
+		t.Parallel()
+		path, cleanup := setupSocketServer(t)
+		defer cleanup()
+
+		ctx := testutil.Context(t, testutil.WaitShort)
+
+		// Register some units in various states.
+		client, err := agentsocket.NewClient(ctx, agentsocket.WithPath(path))
+		require.NoError(t, err)
+		// unit-a: started
+		err = client.SyncStart(ctx, "unit-a")
+		require.NoError(t, err)
+		// unit-b: completed
+		err = client.SyncStart(ctx, "unit-b")
+		require.NoError(t, err)
+		err = client.SyncComplete(ctx, "unit-b")
+		require.NoError(t, err)
+		// unit-c: pending (has unsatisfied dependency on unit-a completing)
+		err = client.SyncWant(ctx, "unit-c", "unit-a")
+		require.NoError(t, err)
+		client.Close()
+
+		var outBuf bytes.Buffer
+		inv, _ := clitest.New(t, "exp", "sync", "list", "--socket-path", path)
+		inv.Stdout = &outBuf
+		inv.Stderr = &outBuf
+
+		err = inv.WithContext(ctx).Run()
+		require.NoError(t, err)
+
+		clitest.TestGoldenFile(t, "TestSyncCommands_Golden/list_with_units", outBuf.Bytes(), nil)
+	})
+
+	t.Run("list_json_format", func(t *testing.T) {
+		t.Parallel()
+		path, cleanup := setupSocketServer(t)
+		defer cleanup()
+
+		ctx := testutil.Context(t, testutil.WaitShort)
+
+		// Register a unit.
+		client, err := agentsocket.NewClient(ctx, agentsocket.WithPath(path))
+		require.NoError(t, err)
+		err = client.SyncStart(ctx, "my-unit")
+		require.NoError(t, err)
+		client.Close()
+
+		var outBuf bytes.Buffer
+		inv, _ := clitest.New(t, "exp", "sync", "list", "--output", "json", "--socket-path", path)
+		inv.Stdout = &outBuf
+		inv.Stderr = &outBuf
+
+		err = inv.WithContext(ctx).Run()
+		require.NoError(t, err)
+
+		clitest.TestGoldenFile(t, "TestSyncCommands_Golden/list_json_format", outBuf.Bytes(), nil)
+	})
 }