feat(submodule): add git-layer primitives and transactional/recursive handlers

[ed-irl]

Introduce the building blocks for submodule-aware git-spice operations: low-level git inspection/snapshot helpers and a new internal/handler/submodule package coordinating them.

• git: add submodule inspection helpers on Worktree — SubmoduleStatus (branch/detached, HEAD hash, parent gitlink hash, and whether the sub has a gs store), plus the underlying SubmoduleHead, SubmoduleGitlink, and SubmoduleHasGsStore.
• git: add HeadSnapshot with SnapshotHead/RestoreHead to capture and restore a worktree's HEAD (branch or detached), used to make multi-sub operations reversible.
• handler/submodule (Applier): ApplyAssociations switches each tracked submodule to the branch recorded for a parent branch, in deterministic order, snapshotting each sub's HEAD and rolling back all switched subs on the first failure. Honors an Exclude list and treats unrecorded/untracked branches as no-ops.
• handler/submodule (recurse): OpenContext builds the per-submodule git-spice plumbing (worktree, repo, store, service) rooted at a sub, returning ErrSubmoduleNotInitialized when the sub lacks a gs store; ForEachInitializedSubmodule iterates .gitmodules, soft-skipping excluded and uninitialized subs.
• handler/submodule (errors): DivergedFromRecordError and FoldConflictError, each rendering copy-pasteable remediation hints.
• git tests: replace inline filepath.ToSlash(filepath.Join(...)) with a joinSlash helper.

Part of #1246

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat(submodule): track and manage submodule branch associations #1178
    • feat(submodule): add git-layer primitives and transactional/recursive handlers #1179 ◀
      • submodule: associations follow checkout, inherit on create, merge on fold #1180
        • feat(worktree): add worktree commands, per-worktree trunks, and worktree filtering #1181
          • feat(worktree): add worktree commands, per-worktree trunks, and worktree-aware filtering #1182
            • feat(worktree): Add worktree commands, per-worktree trunks, and worktree-aware filtering #1183

_{Change managed by git-spice.}