feat: add actions management commands (#796)

## Summary

This PR adds comprehensive Actions secrets and variables management functionality to the tea CLI, enabling users to manage their repository's CI/CD configuration directly from the command line.

## Features Added

### Actions Secrets Management
- **List secrets**: `tea actions secrets list` - Display all repository action secrets
- **Create secrets**: `tea actions secrets create <name>` - Create new secrets with interactive prompts
- **Delete secrets**: `tea actions secrets delete <name>` - Remove existing secrets

### Actions Variables Management
- **List variables**: `tea actions variables list` - Display all repository action variables
- **Set variables**: `tea actions variables set <name> <value>` - Create or update variables
- **Delete variables**: `tea actions variables delete <name>` - Remove existing variables

## Implementation Details

- **Interactive prompts**: Secure input handling for sensitive secret values
- **Input validation**: Proper validation for secret/variable names and values
- **Table formatting**: Consistent output formatting with existing tea commands
- **Error handling**: Comprehensive error handling and user feedback
- **Test coverage**: Full test suite for all functionality

## Usage Examples

```bash
# Secrets management
tea actions secrets list
tea actions secrets create API_KEY    # Will prompt securely for value
tea actions secrets delete OLD_SECRET

# Variables management
tea actions variables list
tea actions variables set API_URL https://api.example.com
tea actions variables delete UNUSED_VAR
```

## Related Issue

Resolves #797

## Testing

- All new functionality includes comprehensive unit tests
- Integration with existing tea CLI patterns and conventions
- Validated against Gitea Actions API

Reviewed-on: https://gitea.com/gitea/tea/pulls/796
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Ross Golder <ross@golder.org>
Co-committed-by: Ross Golder <ross@golder.org>

README.md

@@ -42,6 +42,7 @@ COMMANDS:
     organizations, organization, org  List, create, delete organizations
     repos, repo                       Show repository details
     branches, branch, b               Consult branches
+    actions                           Manage repository actions (secrets, variables)
     comment, c                        Add a comment to an issue / pr
 
   HELPERS:
@@ -77,6 +78,11 @@ EXAMPLES
   tea open 189                        # open web ui for issue 189
   tea open milestones                 # open web ui for milestones
 
+  tea actions secrets list            # list all repository action secrets
+  tea actions secrets create API_KEY  # create a new secret (will prompt for value)
+  tea actions variables list          # list all repository action variables
+  tea actions variables set API_URL https://api.example.com
+
   # send gitea desktop notifications every 5 minutes (bash + libnotify)
   while :; do tea notifications --mine -o simple | xargs -i notify-send {}; sleep 300; done