Clarify license header applies to tests and scripts

The "Source license headers" section previously said "New source
files" without elaborating.  Expand that sentence to make explicit
that the requirement covers test files (*.test.ts) and management
scripts under scripts/ as well as regular source files.

Assisted-by: Claude Code:claude-sonnet-4-6

Author: dahlia

CONTRIBUTING.md

@@ -179,9 +179,10 @@ release, such as lockfile updates or changelog edits.
 Source license headers
 ----------------------
 
-DrFed is licensed under the [GNU Affero General Public License v3].  New source
-files must start with the existing AGPL header.  For TypeScript, JavaScript,
-*.mjs*, and *.mts* files, use this form before imports:
+DrFed is licensed under the [GNU Affero General Public License v3].  Every new
+source file (including test files matching *.test.ts* and management scripts
+under *scripts/*) must start with the existing AGPL header.  For TypeScript,
+JavaScript, *.mjs*, and *.mts* files, use this form before imports:
 
 ~~~~ ts
 // DrFed: A web-based platform for developing and debugging ActivityPub apps