chore: vendor ruvnet/metaharness as a git submodule

Adds vendor/metaharness (github.com/ruvnet/metaharness, pinned at
87b6c51 on main) -- the real MetaHarness generator: repo-aware CLI,
Rust/WASM+NAPI-RS kernel, host adapters, @metaharness/router
(cost-optimal model routing) and @metaharness/darwin ("Darwin Mode",
self-evolving harness config). ADR-285's harness/homecore/ already
depends on the real @metaharness/kernel@0.1.2 npm package; this
vendors the source repo alongside the other vendor/* submodules
(rufield, rvcsi, ruvector, sublinear-time-solver, midstream) for local
inspection and building.

No wiring into any harness/ or crate yet -- follow-up work.

.gitmodules

@@ -29,3 +29,7 @@
 	path = v2/crates/worldgraph
 	url = https://github.com/ruvnet/worldgraph.git
 	branch = main
+[submodule "vendor/metaharness"]
+	path = vendor/metaharness
+	url = https://github.com/ruvnet/metaharness
+	branch = main