docs(specs): spec S17 phase 6 virtual and remote repositories

Why

Phase 6 is the next S17 phase: virtual and remote repositories on the management API. The remote slices are mid-implementation with no user-reachable create (create.go answers 422 to any non-hosted kind), and the monolith CRUD slice needs this contract to extend its client. The patch specs the full surface: non-hosted CRUD with the first settings contents, virtual upstream associations, the connection test, remote cache reads, and eviction semantics on the Phase 4 delete routes.

The review decisions live in Resolutions; the load-bearing ones are that the virtual slices keep their table migrations, create never probes the upstream (monolith/S07 amendment follow-up), deleting a virtual's upstream is a 409 until dissociated, and remote delete_all sweeps one pass with no boundary predicate.

Related to S17 Phase 6: virtual and remote repository CRUD (#314) • Hayley Swimelar.

Test plan

Docs-only. markdownlint, Vale, and lychee pass in pre-commit; all internal and cross-document anchors verified to resolve.