feat: add additional admin users subcommands (#842)

## Summary

Adds admin user management commands to the tea CLI, enabling admins to create, edit, and delete user accounts.

## Features Added

### Admin User Management Commands

- **Create users**: `tea admin users create` - Create new user accounts with configurable options
- **Edit users**: `tea admin users edit <username>` - Update user properties including password, permissions, and profile settings
- **Delete users**: `tea admin users delete <username>` - Remove user accounts with confirmation prompt

### Implementation Details

#### Create Command (`admin users create`)
- Required: username
- Optional: email, full name, password
- Flags: admin, restricted, prohibit-login, visibility
- Password input: command-line flag, file, stdin, or interactive prompt with confirmation
- Default: users must change password on first login (use `--no-must-change-password` to skip)
- Post-creation updates for admin/restricted/prohibit-login (not available during creation)

#### Edit Command (`admin users edit`)
- Updates only explicitly provided fields (partial updates)
- Password change support with the same input methods as create
- Editable fields:
  - Profile: email, full name, description, website, location
  - Permissions: admin/restricted/active status
  - Settings: visibility, max repo creation limits
  - Advanced: git hooks, local imports, organization creation
- Default: password changes require password change on next login (use `--no-must-change-password` to skip)

#### Delete Command (`admin users delete`)
- Confirmation prompt by default
- `--confirm` flag to skip confirmation
- Displays user details before deletion

### Security Features

- Secure password input via interactive prompts (hidden input)
- Multiple password input methods: flag, file, stdin, interactive
- Password confirmation for interactive mode
- Whitespace trimming for file/stdin inputs

### Password Input Methods

1. **Command-line flag**: `--password <value>`
2. **File input**: `--password-file <file>` - Read from file
3. **Stdin input**: `--password-stdin` - Read from stdin
4. **Interactive prompt**: Automatically prompts if password not provided (with confirmation)

For edit command: Use `--password=""` to trigger interactive prompt.

## Usage Examples

```bash
# Create a new user
tea admin users create --username john --email john@example.com --admin --no-must-change-password

# Create with interactive password prompt
tea admin users create jane --email jane@example.com

# Edit user properties
tea admin users edit john --email newemail@example.com --restricted

# Change user password (will prompt if not provided)
tea admin users edit john --password=""
tea admin users edit john --password-file /path/to/password.txt

# Delete a user (with confirmation)
tea admin users delete olduser

# Delete without confirmation
tea admin users delete olduser --confirm
```

## Related Issue

Resolves #161

## Testing

- Unit tests for all commands
- Flag validation and default value tests
- Password input method tests (file, stdin, interactive)
- Test coverage for all user option structures
- Confirmation logic tests for delete command

## Technical Details

- Uses Gitea SDK `AdminCreateUser`, `AdminEditUser`, and `AdminDeleteUser` APIs
- Follows existing tea CLI patterns and conventions
- Handles fields not available during creation via post-creation updates
- Partial update support for edit command (only updates explicitly set fields)
- Consistent with other tea commands (webhooks, secrets) in password handling and confirmation patterns

All tests pass and the implementation integrates with existing tea CLI infrastructure.

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/tea/pulls/842
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: ghainer <gehainer@gmail.com>
Co-committed-by: ghainer <gehainer@gmail.com>

cmd/admin/users/delete.go

@@ -0,0 +1,77 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package users
+
+import (
+	stdctx "context"
+	"fmt"
+
+	"code.gitea.io/tea/cmd/flags"
+	"code.gitea.io/tea/modules/context"
+
+	"github.com/urfave/cli/v3"
+)
+
+// CmdUserDelete represents a sub command of users to delete a user
+var CmdUserDelete = cli.Command{
+	Name:        "delete",
+	Aliases:     []string{"rm", "remove"},
+	Usage:       "Delete a user",
+	Description: "Delete a user account",
+	ArgsUsage:   "<username>",
+	Action:      RunUserDelete,
+	Flags: append([]cli.Flag{
+		&cli.BoolFlag{
+			Name:    "confirm",
+			Aliases: []string{"y"},
+			Usage:   "confirm deletion without prompting",
+		},
+	}, flags.AllDefaultFlags...),
+}
+
+// RunUserDelete deletes a user
+func RunUserDelete(_ stdctx.Context, cmd *cli.Command) error {
+	ctx, err := context.InitCommand(cmd)
+	if err != nil {
+		return err
+	}
+
+	if ctx.Args().Len() == 0 {
+		return fmt.Errorf("username is required")
+	}
+
+	client := ctx.Login.Client()
+	username := ctx.Args().First()
+
+	// Get user details first to show what we're deleting
+	user, _, err := client.GetUserInfo(username)
+	if err != nil {
+		return fmt.Errorf("failed to get user info: %w", err)
+	}
+
+	if !ctx.Bool("confirm") {
+		userInfo := fmt.Sprintf("%s (ID: %d)", user.UserName, user.ID)
+		if user.Email != "" {
+			userInfo += fmt.Sprintf(" - %s", user.Email)
+		}
+		if user.IsAdmin {
+			userInfo += " [admin]"
+		}
+		fmt.Printf("Are you sure you want to delete user %s? [y/N] ", userInfo)
+		var response string
+		fmt.Scanln(&response)
+		if !isConfirmationAccepted(response) {
+			fmt.Println("Deletion canceled.")
+			return nil
+		}
+	}
+
+	_, err = client.AdminDeleteUser(username)
+	if err != nil {
+		return fmt.Errorf("failed to delete user: %w", err)
+	}
+
+	fmt.Printf("User '%s' deleted successfully\n", username)
+	return nil
+}