refactor: extract VersionControlProvider abstraction

[pfitz]

Summary

Pure refactor that extracts a VersionControlProvider interface from sandcastle's git-shaped code. Zero behavior change for existing callers.

The change adds an optional vcs option to run(), interactive(), createSandbox(), and createWorktree(), defaulting to a git() implementation that mirrors current behavior. The interface is the seam through which alternative VCS backends (such as Jujutsu, in a follow-up PR) can be implemented.

Motivation

I personally use Jujutsu (jj) for development and would like to use jj workspaces instead of git worktrees with sandcastle. This PR is the foundation refactor; a follow-up PR will add a `JjProvider` for jj-colocated repos. I figured a proper pluggable abstraction would benefit other users of alternative VCS tools too.

What's in this PR

• `src/VersionControl.ts` — new `VersionControlProvider` interface, one method per logical VCS operation
• `src/vcs/git.ts` — `git()` factory implementing the interface; methods delegate to existing `WorktreeManager`, `syncIn`/`syncOut` helpers, and `resolveGitMounts`
• `src/vcs/git.test.ts` — 30 tests covering the provider against real git in temp repos
• All call sites in `SandboxFactory.ts`, `SandboxLifecycle.ts`, `syncIn.ts`, `syncOut.ts`, `RecoveryMessage.ts`, `Orchestrator.ts`, `createSandbox.ts`, `createWorktree.ts`, `interactive.ts`, `run.ts` updated to dispatch through the new `vcs` option
• Subpath export: `import { git } from "@ai-hero/sandcastle/vcs/git"`
• Patch-level changeset

What's NOT in this PR (deferred to PR 2)

• The `JjProvider` itself
• jj integration tests / CI changes
• Interface JSDoc clarifying jj-specific contracts on `headRef`, `currentBranch`, `hasUncommittedChanges`

A design plan with the full feasibility analysis is committed at `docs/superpowers/plans/2026-05-05-vcs-abstraction-pr1-refactor.md`. Happy to drop it from the diff if you'd prefer a smaller PR — let me know.

Notes on the diff size

• Most new lines are in test code (`src/vcs/git.test.ts` ≈ 480 lines, 30 tests) and the design plan markdown (~910 lines).
• The actual abstraction is ~250 lines (`VersionControl.ts` + `vcs/git.ts`).
• Net call-site refactoring is mechanical — replacing direct `WorktreeManager.create(...)`-style calls with `vcs.createCheckout(...)` etc.

Test plan

• `npm run typecheck` passes (verified locally — clean)
• `npm test` passes (existing tests unchanged, 30 new tests in `git.test.ts` all pass)
• `npm run build` produces `dist/vcs/git.{js,d.ts}` and `dist/VersionControl.{js,d.ts}` (verified)
• Importing `@ai-hero/sandcastle/vcs/git` works after publish

Security note

Several command-builder methods in `git.ts` produce shell command strings that get executed via `handle.exec(...)` inside sandboxes. All user-influenced values are escaped with a POSIX single-quote helper (`shellSingleQuote`) to prevent shell injection. The helper is exported for reuse by future backends.

[vercel]

Someone is attempting to deploy a commit to the Matt Pocock's projects Team on Vercel.

A member of the Team first needs to authorize it.