refactor(cli): standardize dataset command format and remove redundant hash command

Standardized dataset.zig with proper doc comment format:
- Added /// doc comment with usage and subcommand descriptions
- Follows same format as other commands

Removed dataset_hash.zig:
- Hash computation is already automatic in 'dataset verify'
- Standalone 'ml dataset hash' command was redundant
- Users can use 'ml dataset verify <path>' to get hash

All tests pass.

cli/src/commands/dataset_hash.zig

@@ -1,49 +0,0 @@
-const std = @import("std");
-const cli = @import("../../main.zig");
-const hash_mod = @import("../../utils/hash.zig");
-const io = @import("../../utils/io.zig");
-
-pub const name = "dataset hash";
-pub const description = "Hash a dataset directory using SHA256";
-
-pub fn run(allocator: std.mem.Allocator, args: []const []const u8) !void {
-    // Parse arguments
-    if (args.len < 1) {
-        std.debug.print("Usage: ml dataset hash <path>\n", .{});
-        std.debug.print("  <path>  Path to dataset directory\n", .{});
-        return;
-    }
-
-    const path = args[0];
-
-    // Hash the directory
-    io.printInfo("Hashing dataset at: {s}\n", .{path});
-
-    const hash = hash_mod.hashDirectoryToHex(allocator, path) catch |err| {
-        switch (err) {
-            error.PathTraversalAttempt => {
-                io.printError("Invalid path (path traversal detected): {s}\n", .{path});
-            },
-            error.NotAFile => {
-                io.printError("Not a regular file: {s}\n", .{path});
-            },
-            error.EmptyDirectory => {
-                io.printError("Directory is empty or contains no files: {s}\n", .{path});
-            },
-            error.MaxDepthExceeded => {
-                io.printError("Max directory depth exceeded (32): {s}\n", .{path});
-            },
-            error.OutOfMemory => {
-                io.printError("Out of memory\n", .{});
-            },
-            else => {
-                io.printError("Hash computation failed: {s}\n", .{@errorName(err)});
-            },
-        }
-        return err;
-    };
-    defer allocator.free(hash);
-
-    // Print result
-    io.printSuccess("Dataset hash: {s}\n", .{hash});
-}