feat: add repository webhook management (#798)

## Summary

This PR adds support for organization-level and global webhooks in the tea CLI tool.

## Changes Made

### Organization Webhooks
- Added `--org` flag to webhook commands to operate on organization-level webhooks
- Implemented full CRUD operations for org webhooks (create, list, update, delete)
- Extended TeaContext to support organization scope

### Global Webhooks
- Added `--global` flag with placeholder implementation
- Ready for when Gitea SDK adds global webhook API methods

### Technical Details
- Updated context handling to support org/global scopes
- Modified all webhook subcommands (create, list, update, delete)
- Maintained backward compatibility for repository webhooks
- Updated tests and documentation

## Usage Examples

```bash
# Repository webhooks (existing)
tea webhooks list
tea webhooks create https://example.com/hook --events push

# Organization webhooks (new)
tea webhooks list --org myorg
tea webhooks create https://example.com/hook --org myorg --events push,pull_request

# Global webhooks (future)
tea webhooks list --global
```

## Testing
- All existing tests pass
- Updated test expectations for new descriptions
- Manual testing of org webhook operations completed

Closes: webhook management feature request
Reviewed-on: https://gitea.com/gitea/tea/pulls/798
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

@@ -44,6 +44,7 @@ COMMANDS:
     branches, branch, b               Consult branches
     actions                           Manage repository actions (secrets, variables)
     comment, c                        Add a comment to an issue / pr
+    webhooks, webhook                 Manage repository webhooks
 
   HELPERS:
     open, o                         Open something of the repository in web browser
@@ -83,6 +84,10 @@ EXAMPLES
   tea actions variables list          # list all repository action variables
   tea actions variables set API_URL https://api.example.com
 
+  tea webhooks list                   # list repository webhooks
+  tea webhooks list --org myorg       # list organization webhooks
+  tea webhooks create https://example.com/hook --events push,pull_request
+
   # send gitea desktop notifications every 5 minutes (bash + libnotify)
   while :; do tea notifications --mine -o simple | xargs -i notify-send {}; sleep 300; done