Resolve the repository from the invoking directory

A task included from the global mise config runs with the config root as its
working directory - $HOME - rather than the directory the user invoked it from.
Deriving the repository from the current directory therefore failed everywhere
except a project-level include, which defeats the point of installing the task
once and using it in every repository.

mise passes the real directory as MISE_ORIGINAL_CWD, so enter it before
resolving the repository, falling back to the current directory when the task
is run directly rather than through mise.

tests/invocation-directory.test.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# shellcheck source-path=SCRIPTDIR
+# shellcheck source=tasks/ai/sbx
+source "$(dirname "${BASH_SOURCE[0]}")/../tasks/ai/sbx"
+
+failures=0
+work="$(mktemp -d)"
+trap 'rm -rf "$work"' EXIT
+
+fail() {
+    printf 'FAIL: %s\n' "$1" >&2
+    failures=$((failures + 1))
+}
+
+mkdir -p "$work/invoked" "$work/elsewhere"
+
+(
+    cd "$work/elsewhere"
+    MISE_ORIGINAL_CWD="$work/invoked"
+    export MISE_ORIGINAL_CWD
+    enter_invocation_directory
+    [[ "$PWD" == "$work/invoked" ]]
+) || fail "did not enter MISE_ORIGINAL_CWD"
+
+(
+    cd "$work/elsewhere"
+    unset MISE_ORIGINAL_CWD
+    enter_invocation_directory
+    [[ "$PWD" == "$work/elsewhere" ]]
+) || fail "did not stay in PWD when MISE_ORIGINAL_CWD is unset"
+
+if (
+    cd "$work/elsewhere"
+    MISE_ORIGINAL_CWD="$work/does-not-exist"
+    export MISE_ORIGINAL_CWD
+    enter_invocation_directory
+) 2>/dev/null; then
+    fail "an unusable MISE_ORIGINAL_CWD was accepted"
+fi
+
+if ((failures)); then
+    printf '%d assertion(s) failed\n' "$failures" >&2
+    exit 1
+fi
+
+printf 'All invocation directory assertions passed.\n'