Add deterministic local Git workflow effects

[taras]

Story

As a workflow author, I want deterministic local Git components to switch branches, stage selected changes, and commit staged content inside a retained Repository or Worktree.

Contract

The provider-neutral component set is:

• Git.Switch for changing or creating the current branch;
• Git.Add for staging explicit paths; and
• Git.Commit for committing exactly the current index.

Git.Checkout may be supplied as a compatible Git-shaped operation where its semantics are explicit, but Git.Switch is the primary branch operation. Git.Commit accepts message as a prop and children as an expandable message; retaining both supports convenient bindings and pre-commit expansion.

Each component is one expansion, one effect, and one Workspace transaction. A commit does not bundle arbitrary child mutations into its transaction: children finish first, then the commit expansion commits the staged index.

Agents remain read-only. They do not supply a privileged authorized-path set; changes reach Git through ordinary durable File and Git.Add effects.

Acceptance

• Every operation uses the contextual Repository or Worktree and has no path prop.
• Git.Switch changes the named branch deterministically and rejects incompatible retained state.
• Git.Add stages only its explicit path inputs and reports the resulting index/tree identity.
• Git.Commit commits only staged content and fails clearly when nothing is staged.
• Child content and a message prop both produce the intended commit message with documented precedence or conflict behavior.
• Replay cannot create a duplicate branch transition, staging result, or commit.
• Interruption after commit creation reconciles the existing commit under the same effect identity.
• Results identify branch, commit, parent, and tree.
• Local Git metadata and the filtered result commit through Commit Workspace mutations and journal results atomically #365's transaction boundary.

Dependencies

• Add named Repository and Worktree composition #293 supplies Repository and Worktree context.
• Commit Workspace mutations and journal results atomically #365 supplies atomic Workspace-local effects.
• PR 📝 Specify retained workflow Workspaces #358 supplies the public Git composition contract.
• End goal: Document adversarial implementation workflow WIP #181.