docs: add comprehensive Copilot instructions for maintainers

- Add .github/copilot-instructions.md (1150+ lines)

- Document all 40+ files analyzed

- Cover architecture, utilities, advanced features

- Exclude .github/ from ZIP downloads (export-ignore)

- Remove .github/ from .gitignore (now tracked)

.gitattributes

@@ -1,2 +1,8 @@
 *.sh text eol=lf
 *.template text eol=lf
+
+# Exclude development files from GitHub ZIP downloads
+# Users don't need these files, but they remain in Git for maintainers
+.github/ export-ignore
+.gitignore export-ignore
+.gitattributes export-ignore