feat(submodule): track and manage submodule branch associations

[ed-irl]

Record which branch each submodule is on when a parent commit is made, and surface those associations in the branch log and through new management commands.

New commands:

• gs branch submodule list shows the submodule branch associations recorded for a branch (defaults to the current branch).
• gs branch submodule repoint <path> changes the submodule branch associated with the current parent branch, defaulting to the submodule's current branch.

Automatic tracking:

• branch create, commit create, and commit amend record each non-excluded submodule's current branch via a new SubmoduleTracker. Detached-HEAD submodules are skipped, and recording failures only warn.
• spice.submodule.exclude lists submodule paths to skip.

Display:

• gs ls and gs ll render associations as [path:branch]; the JSON log adds a submodules field.

State:

• branchState gains an additive, optional submodules map. UpsertRequest.Submodules merges into existing state: non-empty values set an entry, empty strings remove one, and nil leaves it unchanged.

Git layer:

• Add Worktree.Submodules, SubmoduleCurrentBranch, SubmoduleWorktree, and UpdateSubmodulePointer.
• Fix LocalBranches to resolve the real worktree path for submodules with absorbed gitdirs, where git reports %(worktreepath) as the gitdir; previously this caused restacks inside a submodule to be skipped as "checked out in another worktree".

[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.}