refactor: move allowedTools/disallowedTools parsing to parseGitHubContext

- Change allowedTools and disallowedTools from string to string[] in ParsedGitHubContext type
- Parse comma-separated environment variables into arrays in parseGitHubContext function
- Update create-prompt and install-mcp-server to use pre-parsed arrays
- Update all affected test files to use array syntax
- Eliminate duplicate parsing logic across the codebase

test/prepare-context.test.ts

@@ -242,7 +242,7 @@ describe("parseEnvVarsWithContext", () => {
         ...mockPullRequestCommentContext,
         inputs: {
           ...mockPullRequestCommentContext.inputs,
-          allowedTools: "Tool1,Tool2",
+          allowedTools: ["Tool1", "Tool2"],
         },
       });
       const result = prepareContext(contextWithAllowedTools, "12345");