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.

.github/workflows/main.yml

@@ -115,7 +115,7 @@ jobs:
         run: make build-ci
 
       - name: Run smoke tests
-        run: FENCE_TEST_NETWORK=1 ./scripts/smoke_test.sh ./fence
+        run: GREYWALL_TEST_NETWORK=1 ./scripts/smoke_test.sh ./greywall
 
   test-macos:
     name: Test (macOS)
@@ -160,4 +160,4 @@ jobs:
         run: make build-ci
 
       - name: Run smoke tests
-        run: FENCE_TEST_NETWORK=1 ./scripts/smoke_test.sh ./fence
+        run: GREYWALL_TEST_NETWORK=1 ./scripts/smoke_test.sh ./greywall