chore(vendor): add rvcsi as a vendor submodule (github.com/ruvnet/rvcsi)

rvCSI — the edge RF sensing runtime incubated here as `v2/crates/rvcsi-*`
(ADR-095, ADR-096, PR #542) — now has a standalone home at
github.com/ruvnet/rvcsi (9 crates published to crates.io, @ruv/rvcsi on npm,
a Claude Code plugin). This vendors it under `vendor/rvcsi`, alongside
`vendor/ruvector` / `vendor/midstream` / `vendor/sublinear-time-solver`.

Follow-up: migrate the workspace to consume `vendor/rvcsi/crates/rvcsi-*`
and drop the inline `v2/crates/rvcsi-*` copies (kept for now so this change
is a pure addition).

Co-Authored-By: claude-flow <ruv@ruv.net>

.gitmodules

@@ -10,3 +10,7 @@
 	path = vendor/sublinear-time-solver
 	url = https://github.com/ruvnet/sublinear-time-solver
 	branch = main
+[submodule "vendor/rvcsi"]
+	path = vendor/rvcsi
+	url = https://github.com/ruvnet/rvcsi
+	branch = main