refactor(git-submodules): retrofit to the ADR-0020 context contract

Description 480 -> 248 chars, body 1011 -> 347 words. The single
submodules.md splits into setup-and-update, urls-and-config, and removal.

Restores three regressions the first pass introduced: 'repointing' as the
trigger for the URL branch, which had none while the boundary clause
steered those queries to git-remotes; clone and absorbgitdirs in the output
enum, which dispatch still routed to; and status --cached, the flag that
makes the pre-commit pointer gate verifiable.

plugins/git/.apm/skills/git-submodules/references/removal.md

@@ -0,0 +1,32 @@
+---
+topic: submodules
+source_keys:
+  - git-scm-submodule-docs
+---
+
+# Removing and deinitializing a submodule
+
+Both operations are destructive. Confirm with the user before executing either.
+
+## `deinit` is not removal
+
+```bash
+git submodule deinit <path>   # --all for every submodule, -f if locally modified
+```
+
+`deinit` clears the submodule's section from `.git/config` and empties its working tree. The
+`.gitmodules` entry and the gitlink in the superproject's index are untouched, so the submodule is
+still registered and a later `update --init` brings it straight back. Use it to reclaim disk space
+or to reset a broken checkout, not to delete a dependency.
+
+## Full removal, in order
+
+```bash
+git submodule deinit -f <path>    # unregister from .git/config
+git rm <path>                     # drop the .gitmodules entry and the gitlink from the index
+rm -rf .git/modules/<name>/       # stale git dir: not tracked, not cleaned up by git
+git commit -m "chore: remove <name> submodule"
+```
+
+The third step is the one that gets skipped. `.git/modules/<name>/` survives `git rm`, and while it
+is present Git refuses to add a submodule at the same path again.