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/benchmark.yml

@@ -19,7 +19,7 @@ on:
   #   paths:
   #     - "internal/sandbox/**"
   #     - "internal/proxy/**"
-  #     - "cmd/fence/**"
+  #     - "cmd/greywall/**"
 
 permissions:
   contents: read
@@ -72,7 +72,7 @@ jobs:
       - name: Install benchstat
         run: go install golang.org/x/perf/cmd/benchstat@latest
 
-      - name: Build fence
+      - name: Build greywall
         run: make build-ci
 
       - name: Run Go microbenchmarks
@@ -146,7 +146,7 @@ jobs:
       - name: Install benchstat
         run: go install golang.org/x/perf/cmd/benchstat@latest
 
-      - name: Build fence
+      - name: Build greywall
         run: make build-ci
 
       - name: Run Go microbenchmarks