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/orchestrator-contract.md

@@ -0,0 +1,16 @@
+# Orchestrator request contract
+
+`git-orchestrate` and other calling agents send this shape. The result shape they parse back is in
+`SKILL.md` Step 5, because every run emits one.
+
+Request:
+
+```json
+{
+  "action": "create|switch|delete|rename|track|list|get-intent",
+  "branch": "<branch-name>",
+  "base": "<base branch, optional, defaults to config>",
+  "intent": "<human-readable intent, optional>",
+  "confirm": "<true for destructive ops, omit for read ops>"
+}
+```