rename Fence to Greywall as GreyHaven sandboxing component

Rebrand the project from Fence to Greywall, the sandboxing layer of the
GreyHaven platform. This updates:

- Go module path to gitea.app.monadical.io/monadical/greywall
- Binary name, CLI help text, and all usage examples
- Config paths (~/.config/greywall/greywall.json), env vars (GREYWALL_*)
- Log prefixes ([greywall:*]), temp file prefixes (greywall-*)
- All documentation, scripts, CI workflows, and example files
- README rewritten with GreyHaven branding and Fence attribution

Directory/file renames: cmd/fence → cmd/greywall, pkg/fence → pkg/greywall,
docs/why-fence.md → docs/why-greywall.md, example JSON files, and banner.

examples/01-dev-server/README.md

@@ -1,6 +1,6 @@
 # Dev Server + Redis Demo
 
-This demo shows how fence controls network access: allowing specific external domains while blocking (or allowing) localhost connections.
+This demo shows how greywall controls network access: allowing specific external domains while blocking (or allowing) localhost connections.
 
 ## Prerequisites
 
@@ -21,7 +21,7 @@ npm install
 This shows that requests to Redis (local service) works, but external requests are blocked.
 
 ```bash
-fence -p 3000 --settings fence-external-blocked.json npm start
+greywall -p 3000 --settings greywall-external-blocked.json npm start
 ```
 
 Test it:
@@ -39,7 +39,7 @@ curl http://localhost:3000/api/external
 This shows the opposite: whitelisted external domains work, but Redis (localhost) is blocked.
 
 ```bash
-fence -p 3000 --settings fence-external-only.json npm start
+greywall -p 3000 --settings greywall-external-only.json npm start
 ```
 
 You will immediately notice that Redis connection is blocked on app startup:
@@ -62,8 +62,8 @@ curl http://localhost:3000/api/users
 
 | Config | Redis (localhost) | External (httpbin.org) |
 |--------|-------------------|------------------------|
-| `fence-external-blocked.json` | ✓ Allowed | ✗ Blocked |
-| `fence-external-only.json` | ✗ Blocked | ✓ Allowed |
+| `greywall-external-blocked.json` | ✓ Allowed | ✗ Blocked |
+| `greywall-external-only.json` | ✗ Blocked | ✓ Allowed |
 
 ## Key Settings
 
@@ -75,7 +75,7 @@ curl http://localhost:3000/api/users
 
 ## Note: Node.js Proxy Support
 
-Node.js's native `http`/`https` modules don't respect proxy environment variables. This demo uses [`undici`](https://github.com/nodejs/undici) with `ProxyAgent` to route requests through fence's proxy:
+Node.js's native `http`/`https` modules don't respect proxy environment variables. This demo uses [`undici`](https://github.com/nodejs/undici) with `ProxyAgent` to route requests through greywall's proxy:
 
 ```javascript
 import { ProxyAgent, fetch } from "undici";
@@ -86,4 +86,4 @@ const response = await fetch(url, {
 });
 ```
 
-Without this, external HTTP requests would fail with connection errors (the sandbox blocks them) rather than going through fence's proxy.
+Without this, external HTTP requests would fail with connection errors (the sandbox blocks them) rather than going through greywall's proxy.

examples/01-dev-server/app.js

@@ -2,7 +2,7 @@
  * Demo Express app that:
  * 1. Serves an API on port 3000
  * 2. Connects to Redis on localhost:6379
- * 3. Attempts to call external APIs (blocked by fence)
+ * 3. Attempts to call external APIs (blocked by greywall)
  *
  * This demonstrates allowLocalOutbound - the app can reach
  * local services (Redis) but not the external internet.
@@ -60,7 +60,7 @@ async function fetchExternal(url) {
     signal: AbortSignal.timeout(5000),
   };
 
-  // Use proxy if available (set by fence)
+  // Use proxy if available (set by greywall)
   if (proxyUrl) {
     options.dispatcher = new ProxyAgent(proxyUrl);
   }
@@ -84,7 +84,7 @@ app.get("/", (req, res) => {
       "/api/users": "List all users from Redis",
       "/api/users/:id": "Get user by ID from Redis",
       "/api/health": "Health check",
-      "/api/external": "Try to call external API (blocked by fence)",
+      "/api/external": "Try to call external API (blocked by greywall)",
     },
   });
 });
@@ -160,20 +160,20 @@ app.get("/api/external", async (req, res) => {
 
   try {
     const result = await fetchExternal("https://httpbin.org/get");
-    // Check if we're using a proxy (indicates fence is running)
+    // Check if we're using a proxy (indicates greywall is running)
     const usingProxy = !!(process.env.HTTPS_PROXY || process.env.HTTP_PROXY);
     res.json({
       status: "success",
       message: usingProxy
         ? "✓ Request allowed (httpbin.org is whitelisted)"
-        : "⚠️ No proxy detected - not running in fence",
+        : "⚠️ No proxy detected - not running in greywall",
       proxy: usingProxy ? process.env.HTTPS_PROXY : null,
       data: result,
     });
   } catch (error) {
     res.json({
       status: "blocked",
-      message: "✓ External call blocked by fence",
+      message: "✓ External call blocked by greywall",
       error: error.message,
     });
   }

examples/01-dev-server/package.json

@@ -1,7 +1,7 @@
 {
   "name": "dev-server-demo",
   "version": "1.0.0",
-  "description": "Demo: Dev server with Redis in fence sandbox",
+  "description": "Demo: Dev server with Redis in greywall sandbox",
   "type": "module",
   "main": "app.js",
   "scripts": {