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.

.goreleaser.yaml

@@ -18,8 +18,8 @@ builds:
       - -X main.version={{.Version}}
       - -X main.buildTime={{.Date}}
       - -X main.gitCommit={{.Commit}}
-    binary: fence
-    main: ./cmd/fence
+    binary: greywall
+    main: ./cmd/greywall
 
 archives:
   - formats: [tar.gz]
@@ -77,7 +77,7 @@ changelog:
 
 release:
   github:
-    owner: Use-Tusk
-    name: fence
+    owner: monadical
+    name: greywall
   draft: false
   prerelease: auto