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/permissions.test.ts

@@ -62,8 +62,8 @@ describe("checkWritePermissions", () => {
     inputs: {
       triggerPhrase: "@claude",
       assigneeTrigger: "",
-      allowedTools: "",
-      disallowedTools: "",
+      allowedTools: [],
+      disallowedTools: [],
       customInstructions: "",
       directPrompt: "",
     },