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/references/README.md

@@ -0,0 +1,15 @@
+---
+metadata:
+  source_keys:
+    - git-scm-submodule-docs
+---
+
+# References
+
+## submodules.md
+
+Deep-dive reference: full `update` flag table, workflow patterns (clone, add, keep-pinned, update-to-latest, override URL), the complete safe-removal sequence, `absorbgitdirs`, and `foreach` shell variables. Load when SKILL.md's condensed Operations list isn't enough detail.
+
+## sources.md
+
+Research sources that informed this skill — provenance chain for git-scm-submodule-docs reference material.