feat: add wiki management tools (#95)

Fix #94

## Summary
This PR adds wiki management support to gitea-mcp adding new tools: creating, reading, updating, and deleting wiki pages.

## Changes
- Added `operation/wiki/wiki.go` with wiki tools
- Updated `operation/operation.go` to register it
- Updated `README.md`

## New Tools
- `list_wiki_pages` - List all wiki pages in a repository
- `get_wiki_page` - Get wiki page content and metadata
- `get_wiki_revisions` - Get revision history of a wiki page
- `create_wiki_page` - Create a new wiki page
- `update_wiki_page` - Update an existing wiki page
- `delete_wiki_page` - Delete a wiki page

## Implementation Details
- Uses direct HTTP calls to Gitea wiki API endpoints (v1.16.0+)
- Follows existing MCP patterns and error handling
- Includes fallback logic to prevent "unnamed" pages during updates
- Proper base64 content encoding as per Gitea API spec

## Testing
- All 6 tools tested and working correctly
- Error handling validated
- Integration with existing MCP server confirmed
- Made a test repo & simulated a drone construction using Claude Code (in french sorry) at https://git.kernelpanik.fr/Test-Organization/test_wiki_tools/wiki

Ready for review.
Closes #[94]

Co-authored-by: nox <nox@noxen.net>
Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/95
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Bo-Yi Wu (吳柏毅) <appleboy.tw@gmail.com>
Co-authored-by: Thierry PROST <3kynox@noreply.gitea.com>
Co-committed-by: Thierry PROST <3kynox@noreply.gitea.com>

operation/operation.go

@@ -14,6 +14,7 @@ import (
 	"gitea.com/gitea/gitea-mcp/operation/search"
 	"gitea.com/gitea/gitea-mcp/operation/user"
 	"gitea.com/gitea/gitea-mcp/operation/version"
+	"gitea.com/gitea/gitea-mcp/operation/wiki"
 	mcpContext "gitea.com/gitea/gitea-mcp/pkg/context"
 	"gitea.com/gitea/gitea-mcp/pkg/flag"
 	"gitea.com/gitea/gitea-mcp/pkg/log"
@@ -45,6 +46,9 @@ func RegisterTool(s *server.MCPServer) {
 	// Version Tool
 	s.AddTools(version.Tool.Tools()...)
 
+	// Wiki Tool
+	s.AddTools(wiki.Tool.Tools()...)
+
 	s.DeleteTools("")
 }