Authenticate GitHub with gh tokens per repo, so two accounts can share one box

.github/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
           fi
 
       - name: shellcheck
-        run: shellcheck -s bash install.sh gh/gh claude/build-settings
+        run: shellcheck -s bash install.sh gh/gh git/credential-gh-account claude/build-settings
 
       - name: The shells parse it
         run: |

git/credential-gh-account

@@ -0,0 +1,64 @@
+#!/usr/bin/env bash
+# git credential helper that resolves a GitHub account per repository, the way the gh wrapper
+# next door already does: the repo's `gh.account`, or a username git carries in the remote URL.
+#
+# It exists because `gh auth git-credential` -- the helper `gh auth setup-git` installs -- answers
+# only for whichever account is currently active. With one account that is invisible; with two it
+# is wrong half the time, and wrong in the worst way: a read as the other account returns 404, not
+# 403, so the failure looks like the repository not existing rather than the wrong identity.
+#
+# Resolution is read-only, via `gh auth token --user`, and never `gh auth switch`. Switching
+# mutates the shared active account in hosts.yml, so two sessions working in repos belonging to
+# different accounts flip it under each other mid-request. Same reasoning as gh/gh.
+
+set -uo pipefail
+
+die() {
+    printf 'credential-gh-account: %s\n' "$@" >&2
+    exit 1
+}
+
+# store and erase have nothing to do: the tokens live in gh's hosts.yml and gh owns their
+# lifecycle. Anything other than `get` succeeds silently rather than erroring, because git
+# calls all three and a helper that fails on store turns every push into a warning.
+[[ "${1:-}" == get ]] || exit 0
+
+declare -A req=()
+while IFS='=' read -r key value; do
+    [[ -z "$key" ]] && break
+    req["$key"]="$value"
+done
+
+# Scoped deliberately. A helper that answered for every host would shadow whatever the box uses
+# for GitLab or a private forge, and it has no tokens for those to offer in exchange.
+[[ "${req[protocol]:-}" == https && "${req[host]:-}" == github.com ]] || exit 0
+
+# gh may not be on PATH: git runs credential helpers with the environment of whatever invoked
+# git, which for an editor or a scheduled job is not a login shell that sourced zshrc.
+GH_BINARY="$(command -v gh 2>/dev/null)"
+if [[ -z "$GH_BINARY" ]]; then
+    for candidate in /opt/homebrew/bin/gh /usr/local/bin/gh /home/linuxbrew/.linuxbrew/bin/gh; do
+        [[ -x "$candidate" ]] && GH_BINARY="$candidate" && break
+    done
+fi
+[[ -n "$GH_BINARY" ]] || die "gh was not found, so no token can be resolved for github.com." \
+    "  Install it: brew install gh"
+
+# A username in the request wins: it came from the remote URL or an explicit credential.username,
+# both of which are more specific than a repo-wide default. This is also what makes the first
+# clone work, before there is a repo in which to set gh.account -- clone
+# https://ldayton@github.com/... and the account comes along in the URL.
+account="${req[username]:-}"
+[[ -n "$account" ]] || account="$(git config --get gh.account 2>/dev/null)"
+[[ -n "$account" ]] || die "no gh.account set for this repository, and no username in the URL." \
+    "  Set one:  git config gh.account <github-login>" \
+    "  Or clone: git clone https://<github-login>@github.com/owner/repo.git"
+
+# GH_WRAPPER_DISABLE because gh on PATH is this repo's wrapper, and its policy hook has no
+# business adjudicating a credential lookup that git is blocking on.
+token="$(GH_WRAPPER_DISABLE=1 "$GH_BINARY" auth token --user "$account" 2>/dev/null)"
+[[ -n "$token" ]] || die "gh.account=$account, but gh has no token stored for that account." \
+    "  Log in:  gh auth login   (choose $account)" \
+    "  Check:   gh auth status"
+
+printf 'username=%s\npassword=%s\n' "$account" "$token"

git/gitconfig

@@ -16,3 +16,15 @@
 	smudge = git-lfs smudge -- %f
 	process = git-lfs filter-process
 	required = true
+# GitHub authentication goes through gh's stored tokens rather than an ssh key. A key can be
+# registered on exactly one GitHub account, so two accounts on one machine cannot both use one;
+# tokens have no such limit, and `gh auth login` on a fresh box replaces copying key material
+# onto it. See git/credential-gh-account for how the account is chosen.
+#
+# The empty helper first is a reset, not an accident: helpers accumulate across system, global
+# and local config and are tried in order, so without it a system helper (osxkeychain, or a
+# leftover `gh auth setup-git` line) answers first and wins with the wrong account. Scoped to
+# github.com, so helpers for every other host survive it.
+[credential "https://github.com"]
+	helper =
+	helper = "!\"$HOME/source/dotfiles/git/credential-gh-account\""