feat(git-plugin): add complete git workflow automation suite

## Why
The git plugin only covered a partial slice of common git workflows.
This adds the remaining skill set (branches, commits, history, remotes,
submodules, workflow, worktrees) plus a git-orchestrate agent so the
plugin can handle end-to-end git automation instead of a handful of
commands.

## Implementation Notes
Each new skill was validated against its research docs and org
conventions after initial authoring, which surfaced hallucinated
version pins, factual errors, and completeness gaps that were
corrected in the same pass rather than left for follow-up.

## Impact
Bumps the git plugin to 1.3.0.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

plugins/git/skills/git-branches/README.md

@@ -0,0 +1,22 @@
+# git-branches
+
+Manage the full lifecycle of git branches — create, switch, delete, rename, and track feature/hotfix/release branches under GitHub Flow or Gitflow.
+
+## What it does
+
+This skill handles branch operations within the git workflow suite. It creates branches following GitHub Flow or Gitflow conventions (configurable), switches and tracks branches, handles safe deletion with unmerged-work checks, and retrieves branch intent metadata for use by other skills (e.g., commit message context). It returns structured results suitable for agent composition.
+
+## Usage
+
+```
+/git-branches
+```
+
+Describe your branch task: create a feature/hotfix/release branch, switch, delete, rename, or track. The skill will determine the branching pattern (GitHub Flow or Gitflow) from config or repo state and handle safety checks for destructive operations.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `SKILL.md` | Skill instructions for agents |
+| `references/sources.md` | Research sources backing the branching/gitflow guidance |