feat(registry): add generate-registry.sh script and docs

Shell script reads .pkg ZIPs, extracts manifests, and emits a
registry.json for self-hosted package discovery. Docs cover the
JSON format, admin config, and self-hosting setup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/PACKAGE-REGISTRY.md

@@ -0,0 +1,94 @@
+# Package Registry
+
+The package registry lets admins browse and install packages from an external
+JSON index — a lightweight alternative to manually uploading `.pkg` files.
+
+## Registry JSON Format
+
+The registry is a static JSON file matching the `RegistryResponse` struct:
+
+```json
+{
+  "packages": [
+    {
+      "id": "notes",
+      "title": "Notes",
+      "version": "0.8.0",
+      "description": "Markdown notes with backlinks and graph view",
+      "author": "switchboard",
+      "type": "extension",
+      "tier": "core",
+      "download_url": "https://cdn.example.com/pkg/notes.pkg",
+      "size": 48200,
+      "updated_at": "2026-03-20T00:00:00Z"
+    }
+  ]
+}
+```
+
+### Required Fields
+
+| Field          | Description                                    |
+|----------------|------------------------------------------------|
+| `id`           | Package identifier (matches `manifest.json`)   |
+| `title`        | Display name                                   |
+| `version`      | Semver version string                          |
+| `description`  | Short description                              |
+| `download_url` | HTTPS URL to the `.pkg` file (must be HTTPS)   |
+
+### Optional Fields
+
+| Field        | Description                              |
+|--------------|------------------------------------------|
+| `author`     | Package author                           |
+| `type`       | `extension` or `library`                 |
+| `tier`       | `core`, `official`, or `community`       |
+| `size`       | File size in bytes                       |
+| `updated_at` | ISO 8601 timestamp of last update        |
+
+## Configuring the Registry URL
+
+### Via Admin UI
+
+Navigate to **Admin > Settings > Package Registry** and enter the registry URL.
+
+### Via API
+
+```bash
+curl -X PUT /api/v1/admin/settings/package_registry \
+  -H "Content-Type: application/json" \
+  -d '{"value": {"url": "https://cdn.example.com/pkg/registry.json"}}'
+```
+
+## Generating a Registry
+
+Use `scripts/generate-registry.sh` to build a `registry.json` from a
+directory of `.pkg` files:
+
+```bash
+# Default: reads dist/, uses placeholder base URL
+./scripts/generate-registry.sh
+
+# Custom directory and base URL
+./scripts/generate-registry.sh ./my-packages https://cdn.example.com/pkg > registry.json
+```
+
+Requirements: `jq`, `unzip`, and `stat` (GNU or BSD).
+
+## Self-Hosting
+
+Any HTTPS-capable file server works. Upload your `.pkg` files and the
+generated `registry.json` to the same directory, then point the admin
+setting to the `registry.json` URL.
+
+Example with a static file server:
+
+```
+/var/www/packages/
+  registry.json
+  notes.pkg
+  chat.pkg
+  chat-core.pkg
+```
+
+The registry is fetched and cached for 5 minutes on each browse request.