fix(git-branches): restore merging.md's provenance back-reference

references/sources.md credits atlassian-gitflow-tutorial with contributing
the `--no-ff` requirement on Gitflow supporting-branch merges to
references/merging.md, and merging.md:14 does carry that claim — but the
file's own source_keys listed only context7-git-htmldocs, so the chain was
one-directional.

Surfaced by repairing validate-provenance.sh's Contributing-files parser
in the same series; this skill was one of seven whose sources.md the old
parser could not read, so checks 4 and 5 had never run against it.

Refs #99

plugins/git/.apm/skills/git-branches/references/merging.md

@@ -1,6 +1,7 @@
 ---
 source_keys:
   - context7-git-htmldocs
+  - atlassian-gitflow-tutorial
 ---
 
 # Merging one branch into another

plugins/git/skills/git-branches/references/merging.md

@@ -1,6 +1,7 @@
 ---
 source_keys:
   - context7-git-htmldocs
+  - atlassian-gitflow-tutorial
 ---
 
 # Merging one branch into another