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/branch-patterns.md

@@ -0,0 +1,31 @@
+---
+source_keys:
+  - nvie-gitflow-post
+  - atlassian-gitflow-tutorial
+  - gitflow-cheatsheet
+---
+
+# Branch patterns
+
+Which pattern is in play decides the base branch, the branch name prefix, and whether merges are
+allowed to fast-forward. Default to GitHub Flow — simpler, and what CI/CD-oriented repos expect.
+Fall back to Gitflow only when the config says so or the repo already carries `develop` or
+`release/*` branches.
+
+## GitHub Flow
+
+- Base: `main`
+- Feature branches: `feature/<feature-name>` or `fix/<bug-name>`
+- Merge: fast-forward where possible, to keep history linear
+- Delete the branch after merge
+
+## Gitflow
+
+- Base: `main` (production) plus `develop` (integration)
+- Feature branches: `feature/<feature-name>`, cut from `develop`
+- Release branches: `release/X.Y.Z`, cut from `develop`, merged to both `main` and `develop`
+- Hotfix branches: `hotfix/X.Y.Z`, cut from `main`, merged to both `main` and `develop`
+- Merge: always `--no-ff`, so the branch structure survives in the history
+
+The two are not mixable. A `--no-ff` merge into a GitHub Flow repo leaves merge commits nobody
+expects; a fast-forward merge of a Gitflow release branch erases the release boundary.