Test the Git excludes step

Add tests for the step that excludes .claude, .agents and AGENTS.md from git
in the new project: the three paths are ignored and absent from git status
after a run, while framework, .gitmodules and docs/artifact-registry.md stay
visible; no .gitignore is written and nothing is committed; a second run
writes each entry and the comment once and keeps the existing lines, also
when the file has no final newline; nothing is written for a path git
already ignores; a tracked path stays tracked and is named; the step is
skipped without the framework; the dry-run plan lists it.

The existing .env exclusion test also checks its comment line.

Refs #68
Task: MIL-009#4

tests/test-credentials.sh

@@ -291,6 +291,7 @@ echo done'
   local exclude
   exclude="$(cat "$WORK/p/.git/info/exclude")"
   assert_contains "old entry kept" "$exclude" "build/"
+  assert_contains "comment line" "$exclude" "# RepoFoundry: the credentials file of this project"
   assert_eq "entry added once" "1" "$(grep -c '^\.env$' "$WORK/p/.git/info/exclude")"
   assert_eq "old entry still on its own line" "1" "$(grep -c '^build/$' "$WORK/p/.git/info/exclude")"
 }