claudetool/onstart: support .github/copilot-instructions.md as inject file

Update categorizeFile function to recognize .github/copilot-instructions.md
as an injectable file for the agent prompt, allowing GitHub Copilot
guidance to be included in the system prompt.

Fixes #105

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s211c42575972fdbek
diff --git a/claudetool/onstart/analyze.go b/claudetool/onstart/analyze.go
index 4b3fde2..7ffade0 100644
--- a/claudetool/onstart/analyze.go
+++ b/claudetool/onstart/analyze.go
@@ -153,6 +153,11 @@
 		}
 	}
 
+	// GitHub Copilot: https://code.visualstudio.com/docs/copilot/copilot-customization
+	if path == ".github/copilot-instructions.md" {
+		return "inject"
+	}
+
 	// BuildFiles - build and configuration files
 	if strings.HasPrefix(lowerFilename, "makefile") ||
 		strings.HasSuffix(lowerPath, ".vscode/tasks.json") {