fix(scripts): let issue.sh claim work on a write:issue-only token

`claim` is the one step the workflow requires before the first edit, and
it failed outright on a token scoped to the work it does: `me()` calls
`GET /user` purely to name the assignee, and that endpoint needs
read:user. So the documented process was blocked by its own tooling, and
the fallback was to do the assignment, the label and the comment by hand
— which is the half-made claim `claim` exists to prevent.

GITEA_USER short-circuits the lookup, so least privilege is enough. The
lookup stays as the fallback because it is right when the scope is there
and needs no setup. Failure is now actionable and says both remedies,
and it still happens before any of the three halves are mutated.

Closes #130

scripts/issue.sh

@@ -38,8 +38,10 @@
 # Where a body is taken and no --body-file is given, it is read from stdin.
 #
 # Environment:
-#   GITEA_TOKEN   a PAT with write:issue (plus write:repository and read:user,
-#                 which the rest of this repo's tooling reaches for)
+#   GITEA_TOKEN   a PAT with write:issue.  `claim` and `mine` additionally
+#                 need to know your username: set GITEA_USER, or give the
+#                 token read:user and it is looked up.
+#   GITEA_USER    your Gitea login.  Optional; see above.
 #   GITEA_URL     defaults to https://git.ljones.me
 #   GITEA_REPO    defaults to yonlu/yellowjacket
 set -euo pipefail
@@ -81,7 +83,29 @@ read_body() {
 	if [ "$file" = "-" ]; then cat; else cat "$file"; fi
 }
 
-me() { curl -sS -H "Authorization: token $GITEA_TOKEN" "$server/api/v1/user" | python3 "$py" login; }
+# The one lookup in this script that needs a scope beyond write:issue.
+# `GET /user` requires read:user, and it is reached for exactly two reasons:
+# to name the assignee in `claim`, and to filter in `mine`.  A token scoped to
+# the work this script does — write:issue — therefore failed at `claim`, which
+# is the one step the workflow requires before the first edit, so the whole
+# documented process was blocked by its own tooling.
+#
+# GITEA_USER short-circuits it, which is what lets a least-privilege token do
+# the job.  The lookup stays as the fallback because it is right when the
+# scope is there and needs no setup at all.
+me() {
+	if [ -n "${GITEA_USER:-}" ]; then
+		printf '%s' "$GITEA_USER"
+		return
+	fi
+	curl -sS -H "Authorization: token $GITEA_TOKEN" "$server/api/v1/user" |
+		python3 "$py" login ||
+		{
+			echo "issue.sh: could not resolve your username. Set GITEA_USER, or" >&2
+			echo "issue.sh: re-issue GITEA_TOKEN with read:user." >&2
+			exit 1
+		}
+}
 
 label_id() { call GET "/labels?limit=100" | python3 "$py" label-id "$1"; }