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/skills/git-branches/references/comparing-branches.md

@@ -0,0 +1,16 @@
+---
+source_keys:
+  - context7-git-htmldocs
+---
+
+# Comparing two branches
+
+The two-dot and three-dot forms mean different things and are easy to swap by accident — check the
+direction before reporting a result.
+
+- `git log main..feature` — commits on `feature` that are not on `main`.
+- `git log feature..main` — the reverse direction: commits on `main` not on `feature`.
+- `git log --left-right main...feature` — both diverging sets at once (symmetric difference).
+- `git diff main...feature` — the diff from the common ancestor to `feature`'s tip, which is what
+  a reviewer sees, rather than the diff between the two tips.
+- `git merge-base main feature` — print the common ancestor commit.