refactor(git-branches): retrofit to the ADR-0020 context contract

Description 612 -> 273 chars, body 1124 -> 457 words. Branch patterns,
operations, merging, comparison, and the orchestrator contract move to
references/.

Corrects rebase routing in four places: this skill sent rebase to
git-history, which carries no rebase content and disclaims it. Rebase is
git-commits'; cherry-pick and revert stay git-history's. Drops a Step 3
gate on a rebase flow this skill does not have.

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

@@ -0,0 +1,29 @@
+---
+source_keys:
+  - context7-git-htmldocs
+---
+
+# Merging one branch into another
+
+Scope is fast-forward and merge-commit mechanics plus conflict resolution. Rebase belongs to
+`git-commits`; cherry-pick and revert to `git-history`.
+
+- **Fast-forward** — `git merge <branch>` advances the pointer with no merge commit when the
+  target has not diverged.
+- **True merge** — `git merge --no-ff <branch>` forces a merge commit even when a fast-forward is
+  possible. Gitflow requires it on every supporting-branch merge.
+- **Squash merge** — `git merge --squash <branch>` stages the combined diff without committing.
+  Follow it with a `git commit`.
+- **Octopus merge** — `git merge branch-a branch-b branch-c` merges more than two branches at
+  once, but fails outright on any conflict. Use sequential two-way merges when conflicts are
+  likely.
+
+## Conflict resolution
+
+When Git cannot auto-merge it writes conflict markers and stops mid-merge. Run `git status` to
+list the conflicted files, edit each to resolve its markers, then `git add <file>` and
+`git merge --continue`.
+
+- `git merge --abort` restores the pre-merge state.
+- `git mergetool` opens the configured merge tool.
+- `git diff --diff-filter=U` shows only the still-conflicted files.