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/pip-poetry.md

@@ -18,19 +18,19 @@ Goal: allow Python dependency fetching while keeping egress minimal.
 Run:
 
 ```bash
-fence --settings ./fence.json pip install -r requirements.txt
+greywall --settings ./greywall.json pip install -r requirements.txt
 ```
 
 For Poetry:
 
 ```bash
-fence --settings ./fence.json poetry install
+greywall --settings ./greywall.json poetry install
 ```
 
 ## Iterate with monitor mode
 
 ```bash
-fence -m --settings ./fence.json poetry install
+greywall -m --settings ./greywall.json poetry install
 ```
 
 If you use private indexes, add those domains explicitly.