feat: enhance file exclusion capabilities with .bmad-flattenignore support (#531)

- Added support for a new optional `.bmad-flattenignore` file to allow users to specify additional files to exclude from the flattened XML output.
- Updated README and documentation to reflect the new feature and provide examples for usage.
- Modified ignore rules to incorporate patterns from the `.bmad-flattenignore` file after applying `.gitignore` rules.

Benefits:
- Greater flexibility in managing file exclusions for AI model consumption.

tools/flattener/ignoreRules.js

@@ -154,9 +154,11 @@ async function parseGitignore(gitignorePath) {
 async function loadIgnore(rootDir, extraPatterns = []) {
   const ig = ignore();
   const gitignorePath = path.join(rootDir, '.gitignore');
+  const flattenIgnorePath = path.join(rootDir, '.bmad-flattenignore');
   const patterns = [
     ...(await readIgnoreFile(gitignorePath)),
     ...DEFAULT_PATTERNS,
+    ...(await readIgnoreFile(flattenIgnorePath)),
     ...extraPatterns,
   ];
   // De-duplicate