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/sources.md

@@ -0,0 +1,16 @@
+---
+# Research sources referenced by this skill
+# Each entry documents where the skill's guidance came from.
+---
+
+## git-scm-worktree-docs
+
+**Description:** Git SCM official documentation for `git worktree` command — creating, listing, locking, moving, removing, pruning, and repairing linked worktrees; shared vs. per-worktree state; worktree-scoped config.
+
+**Source:** https://git-scm.com/docs/git-worktree
+
+- **Research doc:** plugins/git/docs/research/docs/git/worktrees.md (whole-document reference — covers `## Concept Overview`, `## Key Commands`, `## Workflow Patterns`, `## Common Gotchas`, `## Configuration`)
+
+**Contributing files:**
+- SKILL.md (Concept, Gotchas, Common Operations, Worked Examples, Return Format)
+- references/worktrees.md (full `add` flag table, sparse-checkout, removable media, remote disambiguation, configuration)