fix(store): refuse to remove a store that contains another store (#1880)

* fix(store): refuse to remove a store that contains another store

store remove deleted the target folder recursively after checking only
the target's own metadata. Another registered store living inside that
folder, such as a shared store vendored as a git submodule (a layout
store register accepts), was deleted with it, uncommitted work
included, and its registry entry was left pointing at a missing path.

Refuse with store_remove_contains_registered_store when another
registration's canonical root is inside the folder. The check runs in
the beforeCommit hook, under the registry lock that commits the
removal, which now receives the registrations that will remain.

* docs(store): move nested-store remove refusal to docs-lab

Document store_remove_contains_registered_store on the canonical docs-lab CLI reference instead of legacy docs/cli.md. docs/agent-contract.md keeps the error-code entry.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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Co-authored-by: Clay Good <hi@claygood.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>