Add remote-job.sh: a detached job whose result outlives the session

`ssh host 'cmd | tail -f log | sed -n /ready/q'` reported success on jobs that
never finished, three structural ways: the broker aborts the long connection, a
readiness pattern can match a log header printed before the work starts, and
`sed -n /x/q` exits the instant it matches — tearing down the very command it
was watching. The replacement runs the job detached on the host and writes
NAME.out, NAME.rc, then NAME.done LAST, so status is a cheap read of files over
a short-lived connection. That survives the connection, the restart, and the job
that restarts the endpoint the agent itself is running on.

Black-box tests pin the sentinel ordering (rc before done), that a finished-but-
failed job is DONE rc=N with the report still exiting 0, and that a name with no
state is never reported DONE.

Refs #53

tests/remote-job.bats

@@ -0,0 +1,135 @@
+#!/usr/bin/env bats
+# tests/remote-job.bats — black-box tests for scripts/remote-job.sh (Refs #53)
+#
+# Contract: a job runs detached, writes its output, then its exit code, then touches .done LAST.
+# Status is then a cheap independent read that survives the session that started the job — the
+# whole point is replacing `ssh host 'tail -f | sed'`, which reported success on jobs that had not
+# finished. Exit status answers "is everything finished"; the rc= field answers "did it work".
+#
+# Jobs really run here, against a temp job dir. No fake filesystem state is pre-seeded, because a
+# fixture that pre-creates .done proves nothing about the code that is supposed to create it.
+
+setup() {
+  TEST_TMP="$(mktemp -d)"
+  SCRIPT="$BATS_TEST_DIRNAME/../scripts/remote-job.sh"
+  export REMOTE_JOBS_DIR="$TEST_TMP/jobs"
+}
+
+teardown() {
+  # Jobs are detached on purpose; leave nothing behind that could outlive the test run.
+  pkill -P 1 -f "$TEST_TMP" 2>/dev/null || true
+  rm -rf "$TEST_TMP"
+}
+
+# until_done NAME SECONDS — poll instead of sleeping a fixed amount, so the suite is neither slow
+# nor flaky on a loaded machine.
+until_done() {
+  local name="$1" limit="${2:-10}" waited=0
+  while [[ ! -e "$REMOTE_JOBS_DIR/$name.done" && $waited -lt $((limit * 10)) ]]; do
+    sleep 0.1; waited=$((waited + 1))
+  done
+  [[ -e "$REMOTE_JOBS_DIR/$name.done" ]]
+}
+
+@test "start returns immediately while the job is still running" {
+  run timeout 10 "$SCRIPT" start slow -- sleep 2
+  [ "$status" -eq 0 ]
+  [[ "$output" == "started slow" ]]
+  # A 2-second job must not have blocked a start that is supposed to return at once, and while it
+  # is still in flight the report must say unfinished — never DONE.
+  run "$SCRIPT" status slow
+  [[ "$output" == *RUN* || "$output" == *PENDING* ]]
+  [ "$status" -eq 1 ]
+}
+
+@test "a successful job is reported DONE with its output" {
+  "$SCRIPT" start ok -- bash -c 'echo marker-4711'
+  until_done ok
+  run "$SCRIPT" status ok
+  [[ "$output" == *"ok"*DONE*rc=0* ]]
+  [ "$status" -eq 0 ]
+  run "$SCRIPT" out ok
+  [[ "$output" == *marker-4711* ]]
+}
+
+@test "a failing job's exit code survives into the report" {
+  # This is the reason the .rc file exists: a supervisor that cannot see the failure code is the
+  # bug the runner was written to fix.
+  "$SCRIPT" start boom -- bash -c 'echo it failed here >&2; exit 7'
+  until_done boom
+  run "$SCRIPT" status boom
+  [[ "$output" == *"boom"*DONE*rc=7* ]]
+  run "$SCRIPT" out boom
+  [[ "$output" == *"it failed here"* ]]
+}
+
+@test "the .done sentinel is never written before the exit code" {
+  # If .done landed first, a waiter could read an empty rc and call the job successful.
+  "$SCRIPT" start order -- bash -c 'echo x'
+  until_done order
+  [ -f "$REMOTE_JOBS_DIR/order.rc" ]
+  [ -s "$REMOTE_JOBS_DIR/order.rc" ]
+  run cat "$REMOTE_JOBS_DIR/order.rc"
+  [[ "$output" =~ ^[0-9]+$ ]]
+}
+
+@test "wait prints DONE with the exit code and a tail of the output" {
+  "$SCRIPT" start fin -- bash -c 'echo one; echo two'
+  run "$SCRIPT" wait fin 15
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"DONE fin rc=0"* ]]
+  [[ "$output" == *two* ]]
+}
+
+@test "wait gives up on its own and says so" {
+  "$SCRIPT" start never -- sleep 30
+  run "$SCRIPT" wait never 1
+  [ "$status" -eq 1 ]
+  [[ "$output" == *TIMEOUT*never* ]]
+  "$SCRIPT" status never | grep -q .   # the job is still there, unfinished
+}
+
+@test "status distinguishes running from pending from done" {
+  run "$SCRIPT" status
+  [[ "$output" == *"(no jobs)"* ]]
+  [ "$status" -eq 0 ]
+
+  "$SCRIPT" start later -- sleep 3
+  run "$SCRIPT" status later
+  [[ "$output" == *later* ]]
+  [[ "$output" == *RUN* || "$output" == *PENDING* ]]
+  [ "$status" -eq 1 ]   # unfinished: a supervisor must be able to branch on this
+}
+
+@test "two jobs in flight keep separate state" {
+  "$SCRIPT" start a -- bash -c 'echo A; exit 0'
+  "$SCRIPT" start b -- bash -c 'echo B; exit 2'
+  until_done a && until_done b
+  run "$SCRIPT" status
+  [[ "$output" == *"a"*DONE*rc=0* ]]
+  [[ "$output" == *"b"*DONE*rc=2* ]]
+  run "$SCRIPT" out a
+  [[ "$output" == A ]]
+  run "$SCRIPT" out b
+  [[ "$output" == B ]]
+}
+
+@test "out honours a line limit" {
+  "$SCRIPT" start many -- bash -c 'seq 1 40'
+  until_done many
+  run "$SCRIPT" out many 3
+  [ "$(printf '%s\n' "$output" | grep -c .)" -eq 3 ]
+  [[ "$output" == *38* ]]   # the tail, not the head
+}
+
+@test "an unknown verb is a usage error" {
+  run "$SCRIPT" frobnicate
+  [ "$status" -eq 2 ]
+  [[ "$output" == *usage* ]]
+}
+
+@test "the job directory is created on first use" {
+  [ ! -d "$REMOTE_JOBS_DIR" ]
+  "$SCRIPT" status
+  [ -d "$REMOTE_JOBS_DIR" ]
+}