fix(plugins): move marketplace manifest to repo root for /plugin marketplace add ruvnet/RuView

Claude Code looks for `.claude-plugin/marketplace.json` at the cloned repo's
ROOT — not in a subdirectory — so `/plugin marketplace add ruvnet/RuView`
(and `claude plugin marketplace add ruvnet/RuView`) was failing with
"Marketplace file not found".

- Move `plugins/.claude-plugin/marketplace.json` → `.claude-plugin/marketplace.json`
  (repo root); the `ruview` plugin's `source` is now `./plugins/ruview`.
- README.md / plugins/ruview/README.md: install instructions now use
  `/plugin marketplace add ruvnet/RuView` + `/plugin install ruview@ruview`
  (with `claude --plugin-dir ./plugins/ruview` as the no-install fallback);
  manifest path references updated.
- plugins/ruview/scripts/smoke.sh: resolve the manifest at the repo root;
  also assert the plugin `source` is `./plugins/ruview`.
- ADR-0001 updated (scope, directory contract, smoke contract, consequences).

Verified: `claude plugin validate .` + `./plugins/ruview` pass; smoke 13/13;
`claude plugin marketplace add ./` → `claude plugin install ruview@ruview` →
`claude plugin details ruview` works end-to-end (16 skill-entries + 3 agents).

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

.claude-plugin/marketplace.json

@@ -0,0 +1,15 @@
+{
+  "name": "ruview",
+  "description": "RuView Marketplace: Claude Code + Codex plugins for WiFi sensing — configuration, applications, model training, and onboarding, from practical to advanced",
+  "owner": {
+    "name": "ruvnet",
+    "url": "https://github.com/ruvnet/RuView"
+  },
+  "plugins": [
+    {
+      "name": "ruview",
+      "source": "./plugins/ruview",
+      "description": "End-to-end RuView toolkit: getting started, ESP32 hardware setup, configuration, sensing applications (presence / vitals / pose / sleep / MAT), camera-free + camera-supervised model training, advanced multistatic sensing, CLI / API / WASM, mmWave radar, and witness verification"
+    }
+  ]
+}