Add the design record for distributing gitea-axi as a Nix flake: a
package, a development shell, and a checks output, plus the continuous
integration changes that come with it.
ADR 0018 records the wrapper's deference to the operator's own `git` and
`tea` — the reverse of the hermetic instinct, chosen because `tea`
refreshes OAuth tokens in place and so must not have two versions
mutating one credential store.
Also records the `tea`-is-still-a-runtime-dependency gotcha, which ADR
0002's title obscures.
