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/recipes/git-clone.md

@@ -18,7 +18,7 @@ Goal: allow fetching code from a limited set of hosts.
 Run:
 
 ```bash
-fence --settings ./fence.json git clone https://github.com/OWNER/REPO.git
+greywall --settings ./greywall.json git clone https://github.com/OWNER/REPO.git
 ```
 
 ## SSH clone
@@ -28,5 +28,5 @@ SSH traffic may go through SOCKS5 (`ALL_PROXY`) depending on your git/ssh config
 If it fails, use monitor/debug mode to see what was blocked:
 
 ```bash
-fence -m --settings ./fence.json git clone git@github.com:OWNER/REPO.git
+greywall -m --settings ./greywall.json git clone git@github.com:OWNER/REPO.git
 ```