feat: dev plugin install flow + smoke-platform

- .gitignore: add plugins/ (local dev install, never committed)
- scripts/install-dev-plugins.sh: install dist package from ../verstak-official-plugins/dist/ into ./plugins/
- scripts/smoke-platform.sh: headless verification of plugin discovery, manifest, capabilities, contributions
- cmd/smoke-platform/main.go: Go smoke command for headless plugin verification
- docs/DEV_PLUGINS.md: dev plugin flow documentation

docs/DEV_PLUGINS.md

@@ -0,0 +1,75 @@
+# Verstak Desktop — Development Plugin Flow
+
+## Overview
+
+Official plugins live in the **verstak-official-plugins** monorepo and are developed
+separately from the desktop core. During development, plugins are installed into
+the desktop's local `plugins/` directory as **packaged bundles** (not source code).
+
+```
+git/
+  verstak-desktop/                ← desktop core
+  verstak-official-plugins/       ← official plugin source + dist output
+```
+
+## Plugin Source → Package Flow
+
+1. **Source code** lives in `verstak-official-plugins/plugins/<plugin-id>/`
+2. **Running `build.sh`** in official-plugins:
+   - Builds frontend (if frontend/package.json exists)
+   - Builds backend Go binary (if backend/main.go exists)
+   - Packages the result into `dist/<plugin-id>/`
+3. **Package structure** (`dist/platform-test/`):
+   ```
+   plugin.json
+   frontend/dist/index.js
+   backend/platform-test          (compiled binary)
+   ```
+
+## Installing Dev Plugins in Desktop
+
+From the `verstak-desktop/` directory:
+
+```bash
+./scripts/install-dev-plugins.sh
+```
+
+This script:
+- Locates `../verstak-official-plugins/`
+- Builds packages there if `dist/` is stale
+- Creates `./plugins/<plugin-id>/` from the dist package
+- Does **not** affect other plugins in `./plugins/`
+
+The `plugins/` directory is in `.gitignore` — it is never committed.
+
+## Smoke Test
+
+After installing, verify that the desktop runtime discovers the plugin:
+
+```bash
+./scripts/smoke-platform.sh
+```
+
+This validates:
+- Plugin directory exists
+- `plugin.json` manifest is valid
+- All required manifest fields are present
+- `DiscoverPlugins()` finds `verstak.platform-test`
+- Capabilities are registerable
+
+## Desktop Runtime Scanning Paths
+
+The desktop scans two directories for plugins:
+
+| Path | Purpose |
+|------|---------|
+| `~/.config/verstak/plugins/` | User-installed plugins |
+| `./plugins/` | Bundled/dev plugins (project-local) |
+
+## Important Rules
+
+- **Never commit** `plugins/` to `verstak-desktop` — it's in `.gitignore`
+- **Never copy source code** from `verstak-official-plugins/plugins/` directly —
+  always use the dist package from `verstak-official-plugins/dist/`
+- **Run `install-dev-plugins.sh`** after any change in the plugin source
+- **Run `smoke-platform.sh`** after installing to verify discovery