Submodule support: tracking, checkout, commit, and recursive operations

[ed-irl-codebot]

Problem

git-spice has no awareness of Git submodules. Stacks that change a
submodule pointer can't track which submodule branch a parent commit
expects, there is no help checking out or committing the right
submodule state, and recursive operations across a superproject and its
submodules aren't supported. There is also a latent bug: inside a
submodule with an absorbed gitdir, git-spice mis-resolves the worktree
path and treats every branch as "checked out in another worktree,"
breaking restack and sync.

Proposal

Add submodule support, layered from a tracking baseline up to recursive
operations:

• Tracking + commands: a Submodules map on each tracked branch in
  the store; git-layer listing from .gitmodules, current-branch and
  worktree resolution, and pointer updates; a submodule.Tracker with
  exclusion via spice.submodule.exclude; commit-time recording of the
  per-submodule branch on the parent; gs branch submodule list /
  repoint commands; and [path:branch] annotations in gs ls/ll.
  Also fixes the absorbed-gitdir worktree-path resolution bug.
• Primitives: building blocks for transactional checkout and
  recursive operations.
• Auto behaviors: auto-checkout, branch-create inheritance, and
  branch-fold merge for submodules.
• Commit integration: auto-commit and gitlink pointer updates on
  parent commits.
• Recursive ops: gs repo init --recurse-submodules and
  --recurse-submodules for gs stack restack.

Tracked by

• feat(submodule): track and manage submodule branch associations #1178 — tracking commands + absorbed-gitdir worktree fix (baseline)
• feat(submodule): add git-layer primitives and transactional/recursive handlers #1179 — primitives for transactional checkout and recursive ops
• submodule: associations follow checkout, inherit on create, merge on fold #1180 — auto-checkout, branch-create inheritance, branch-fold merge
• feat(worktree): add worktree commands, per-worktree trunks, and worktree filtering #1181 — auto-commit and gitlink update on parent commits
• feat(worktree): add worktree commands, per-worktree trunks, and worktree-aware filtering #1182 — gs repo init --recurse-submodules
• feat(worktree): Add worktree commands, per-worktree trunks, and worktree-aware filtering #1183 — --recurse-submodules for gs stack restack