agit: init agit helper

This initializes a very simple AGit helper script.

Usage:

```
agit create
```
or
```
agit c
```

To create a new AGit Pr.

Integrate Pulling from an AGit remote.
Gitea doesn't expose an AGit endpoint in the api, or the `tea` cli.
This makes pulling not feasible, since there is no robust way to query
the AGit topic, which is the ref that need to be pulled.

One possible solution currently could be scraping the gitea instructions
for forking a pull request on an AGit PR.

devShell.nix

@@ -33,6 +33,7 @@
           self'.packages.tea-create-pr
           self'.packages.merge-after-ci
           self'.packages.pending-reviews
+          self'.packages.agit
           # treefmt with config defined in ./flake-parts/formatting.nix
           config.treefmt.build.wrapper
         ];