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/sources.md

@@ -0,0 +1,29 @@
+---
+topic: submodules
+source_keys:
+  - git-scm-submodule-docs
+---
+
+## git-scm-submodule-docs
+
+**Description:** Official git-scm.com reference for `git submodule` — all subcommands, flags, configuration keys, and behaviour details.
+
+**Source:** https://git-scm.com/docs/git-submodule
+
+- **Research doc:** plugins/git/docs/research/docs/git/submodules.md (whole-document reference — the research doc is organized by descriptive prose headings such as "Concept Overview" and "Key Commands" rather than a heading matching this slug; this key covers the entire doc, not a single section)
+
+**Contributing files:**
+- SKILL.md (all sections)
+- references/submodules.md (all sections)
+
+---
+
+Other source keys extracted during the git plugin research phase inform sibling skills in the git workflow suite, not this one:
+
+- `context7-git-htmldocs` — git:branches, git:history, git:remotes
+- `git-scm-docs` — git:configuration
+- `git-scm-worktree-docs` — git:worktrees
+- `nvie-gitflow-post`, `atlassian-gitflow-tutorial`, `gitflow-cheatsheet` — git:branches
+- `conventional-commits-spec`, `commitlint-config-conventional` — git:commits
+- `git-scm-push-docs`, `git-scm-fetch-docs`, `git-scm-pull-docs`, `git-scm-remote-docs` — git:remotes
+- `git-scm-bisect-docs`, `git-scm-log-docs`, `git-scm-diff-docs` — git:history