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-worktrees/references/README.md

@@ -0,0 +1,13 @@
+---
+source_keys:
+  - git-scm-worktree-docs
+---
+
+# References
+
+This directory contains provenance metadata and research sources for the `git-worktrees` skill.
+
+## Files
+
+- `sources.md` — Extracted research sources and their contributing documents
+- `worktrees.md` — Full `add` flag table, sparse-checkout setup, removable-media locking, remote-branch disambiguation, and the config key reference