feat(tools): file_read with default-branch resolution

Adds GetFileContents to the gitea client and a file_read MCP tool.
When ref is omitted, the tool resolves the repo default_branch via
GetRepo before fetching contents. Decoded content capped at 1 MiB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/gitea-mcp/main.go

@@ -29,6 +29,7 @@ func main() {
 	reg := registry.New()
 	reg.Register(tools.NewRepoList(giteaClient, ownerAllow))
 	reg.Register(tools.NewRepoGet(giteaClient, ownerAllow))
+	reg.Register(tools.NewFileRead(giteaClient, ownerAllow))
 
 	mcpSrv := mcp.NewServer(mcp.ServerOptions{
 		Registry: reg,