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-submodules/README.md

@@ -0,0 +1,24 @@
+# git-submodules
+
+Initialize, clone, update, and manage git submodules for multi-repository projects.
+
+## What it does
+
+This skill handles submodule operations within the git workflow suite. It initializes submodules, clones repositories with nested submodule dependencies, updates submodule pinning, and manages version control across multi-repo projects. The skill provides clean workflows for projects with complex dependency structures and returns structured results suitable for agent composition.
+
+## Usage
+
+```
+/git-submodules
+```
+
+Describe your submodule task: initialize, clone, update, or manage versions. The skill will handle the operation and return structured results (operation, status, per-submodule details, conflicts, and a recovery `next_step` when applicable) suitable for agent composition.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `SKILL.md` | Skill instructions for agents |
+| `references/README.md` | Describes contents of references/ |
+| `references/submodules.md` | Deep-dive reference: full flag tables, workflow patterns, safe-removal sequence, `absorbgitdirs`, `foreach` variables |
+| `references/sources.md` | Research sources and provenance |