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/README.md

@@ -0,0 +1,24 @@
+# git-worktrees
+
+Manage git worktrees to enable multi-branch parallel development across isolated directories.
+
+## What it does
+
+This skill handles worktree operations within the git workflow suite. It creates, lists, locks/unlocks, moves, removes, prunes, and repairs worktrees — letting an agent work on multiple branches simultaneously without stashing. It returns structured results (paths, branches, lock status) suitable for agent composition.
+
+## Usage
+
+```
+/git-worktrees
+```
+
+Describe your worktree task: create a worktree for a branch, list existing worktrees, lock one for removable media, move, remove, prune, or repair. The skill will handle the operation with appropriate safety checks and return results.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `SKILL.md` | Skill instructions for agents |
+| `references/README.md` | Describes the references directory contents |
+| `references/worktrees.md` | Full `add` flag table, sparse-checkout, removable-media locking, remote disambiguation, configuration |
+| `references/sources.md` | Research sources and provenance |