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/mockContext.ts

@@ -11,8 +11,8 @@ const defaultInputs = {
   triggerPhrase: "/claude",
   assigneeTrigger: "",
   anthropicModel: "claude-3-7-sonnet-20250219",
-  allowedTools: "",
-  disallowedTools: "",
+  allowedTools: [] as string[],
+  disallowedTools: [] as string[],
   customInstructions: "",
   directPrompt: "",
   useBedrock: false,