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/worktrees.md

@@ -0,0 +1,63 @@
+---
+topic: worktrees
+source_keys:
+  - git-scm-worktree-docs
+---
+
+## Full `add` flag table
+
+| Flag | Meaning |
+|---|---|
+| `-b <branch>` | Create and check out a new branch; fails if it exists |
+| `-B <branch>` | Like `-b` but resets the branch if it already exists |
+| `-d` / `--detach` | Detach HEAD; useful for throwaway experiments |
+| `--orphan` | Create empty unborn branch |
+| `--no-checkout` | Suppress initial checkout (for sparse-checkout setup) |
+| `--guess-remote` | Look for a matching remote-tracking branch by path basename |
+| `--lock [--reason <str>]` | Lock immediately on creation (atomic; avoids race vs. add-then-lock) |
+| `-f` / `--force` | Allow when branch is already checked out elsewhere |
+| `--relative-paths` | Link via relative paths (portable across moves) |
+
+Using `-` as `<commit-ish>` is shorthand for `@{-1}` (the branch checked out before the current one), e.g. `git worktree add <path> -`.
+
+## New unborn branch
+
+```bash
+git worktree add --orphan -b <branch> <path>
+```
+Creates an empty branch with no commits.
+
+## Sparse-checkout worktree
+
+Suppress the initial checkout to configure sparse-checkout first:
+```bash
+git worktree add --no-checkout ../sparse main
+cd ../sparse
+git sparse-checkout init --cone
+git sparse-checkout set src/
+git checkout main
+```
+
+## Worktree on removable media
+
+```bash
+git worktree add --lock --reason "external SSD" <path> <branch>
+git worktree unlock <path>  # when reconnected
+```
+
+## Remote-branch disambiguation
+
+```bash
+git worktree add <path> <remote>/<branch>
+```
+For ambiguous names across remotes, `checkout.defaultRemote` config disambiguates explicitly, or `--guess-remote` auto-matches by path basename (default controlled by `worktree.guessRemote` config). If a branch name matches multiple remotes during `worktree add` and neither is set, Git refuses rather than guessing.
+
+## Configuration
+
+| Key | Effect |
+|---|---|
+| `worktree.guessRemote` | Default for `--guess-remote` on `git worktree add` |
+| `worktree.useRelativePaths` | Default for `--relative-paths` on `git worktree add` (link via relative paths — portable across moves) |
+| `gc.worktreePruneExpire` | How long before stale worktree metadata is pruned by `git gc` |
+| `extensions.worktreeConfig` | Enable per-worktree config scope (`config.worktree` file) — see Gotchas in SKILL.md |
+| `checkout.defaultRemote` | Disambiguates which remote to use when a branch name matches multiple remotes during `worktree add` |