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.

docs/templates.md

@@ -1,6 +1,6 @@
 # Config Templates
 
-Fence includes built-in config templates for common use cases. Templates are embedded in the binary, so you can use them directly without copying files.
+Greywall includes built-in config templates for common use cases. Templates are embedded in the binary, so you can use them directly without copying files.
 
 ## Using templates
 
@@ -8,13 +8,13 @@ Use the `-t` / `--template` flag to apply a template:
 
 ```bash
 # Use a built-in template
-fence -t npm-install npm install
+greywall -t npm-install npm install
 
 # Wraps Claude Code
-fence -t code -- claude
+greywall -t code -- claude
 
 # List available templates
-fence --list-templates
+greywall --list-templates
 ```
 
 You can also copy and customize templates from [`internal/templates/`](/internal/templates/).