Every agent becomes a Forgejo user of the same name, and nothing upstream
of `CreateForgeUser` knew what Forgejo refuses: `admin`, `api`, `foo-` or a
41-character name passed name validation and failed one node into
provisioning with Forgejo's 422.
`nix/reserved-names.nix` gains the 22 reserved usernames of Forgejo
v16.0.5 (`models/user/user.go:639-680`) that `[a-z0-9-]` can spell, and
the bare `-` (`models/repo/repo.go:67`). The dot and underscore entries are
left out, since our charset cannot produce them. The header's admission
rule grows a third class — a username the forge refuses — because that is
a failure behind the refusal.
The shape rules are not literals, so they live in
`hive_types::forge_username_violation`: no leading `-`, no `--`, no
trailing `-`, at most 40 characters. Beside `is_reserved_name`, not in
`Ident::parse`: an `Ident` is also a hive, label, account and subagent
name, and parsing runs on every read of an existing name.
`create_agent` used to WARN on a reserved name, deliberately: an operator
with agents already created under a colliding name would otherwise be
unable to re-run creation. That reason is kept, and narrowed to what it
protects. A name breaking either rule is now refused with a 400 naming the
rule when the name is NOT in the swarm roster, and still only warned
about when it is, so re-creating an existing agent keeps working. The
roster is read only for a rule-breaking name; when it cannot be read, a new
name and an existing one look alike, and this warns as before. The
hive-collision warning is unchanged.
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