feat(doctor): --online, for the release a host has never fetched

Everything doctor checks compares copies already on the machine, deliberately —
it runs in a SessionStart hook where latency shows, and its header promises no
network I/O. That leaves exactly one case invisible, and #30 walked straight
into it: a host whose checkout, marketplace clone and installed plugin all agree
reads as clean while all three sit a release behind origin.

`--online` closes it with one `git ls-remote`, opt-in, for a scheduled per-host
job rather than the session hook. Comparison is by order, not equality, so a
host ahead of the newest tag — normal between a release commit and its tag —
stays quiet.

An unreachable remote is a WARN naming itself, not a staleness FAIL. "I could
not check" and "you are behind" are different answers, and collapsing them would
make a network blip look like a stale library.

The remote URL is never printed: a remote rewritten by url.insteadOf expands
into a live token, which is a documented way this fleet has leaked one before.

Five fixture cases through a $DOCTOR_LATEST_TAG_CMD seam, including one that
asserts the DEFAULT run performs no lookup at all — the promise in the header is
now a test, not a comment. Verified live on koala:

  $ bin/doctor --online
  FAIL behind: this host has 0.12.1; origin has released 0.13.4
  $ bin/doctor
  skills doctor: clean — 41 skills

Closes #38
Bump-Type: minor

tests/doctor_test.sh

@@ -224,5 +224,72 @@ manifest "$f/ck" 0.7.0
 manifest "$f/home/.claude/plugins/marketplaces/mathias" 0.9.0
 assert_fails "$f" '0.9.0' 'a newer marketplace clone is reported'
 
+# --- 9. --online: the release this host has never fetched ------------------
+# Everything above compares copies already on the host, so a host where the
+# checkout, the marketplace clone and the installed plugin all agree reads as
+# clean while being a release behind origin. Offline by design (#30); this is
+# the opt-in that closes it (#38).
+#
+# The remote lookup is stubbed through $DOCTOR_LATEST_TAG_CMD so no test needs
+# a network or a real remote.
+
+online_fixture() {
+  _f=$(new_fixture "$1"); wire_clean "$_f"
+  installed_state "$_f" "$2"
+  manifest "$_f/ck" "$2"
+  printf '%s' "$_f"
+}
+
+# assert_online <fixture> <stub-tag> <want-rc> <needle> <label>
+assert_online() {
+  _out=$(HOME="$1/home" DOCTOR_CHECKOUT="$1/ck" \
+         DOCTOR_LATEST_TAG_CMD="printf %s\\n $2" sh "$DOCTOR" --online 2>&1); _rc=$?
+  if [ "$3" -eq 0 ] && [ "$_rc" -ne 0 ]; then
+    fail=$((fail + 1)); printf '  FAIL %s\n       exited %s on a current host\n       got: %s\n' "$5" "$_rc" "$_out"; return
+  fi
+  if [ "$3" -ne 0 ] && [ "$_rc" -eq 0 ]; then
+    fail=$((fail + 1)); printf '  FAIL %s\n       exited 0; staleness went unreported\n' "$5"; return
+  fi
+  if [ -n "$4" ] && ! printf '%s' "$_out" | grep -q -- "$4"; then
+    fail=$((fail + 1)); printf '  FAIL %s\n       report never mentions "%s"\n       got: %s\n' "$5" "$4" "$_out"; return
+  fi
+  pass=$((pass + 1)); printf '  ok   %s\n' "$5"
+}
+
+f=$(online_fixture behind 0.8.0)
+assert_online "$f" v0.9.0 1 '0.9.0' '--online reports a release the host has not fetched'
+
+f=$(online_fixture current 0.9.0)
+assert_online "$f" v0.9.0 0 '' '--online stays quiet when the host is current'
+
+# A host AHEAD of the newest tag is the normal state between a release commit
+# and its tag; that is not drift (same reasoning as check's ordering rule).
+f=$(online_fixture ahead 0.9.1)
+assert_online "$f" v0.9.0 0 '' '--online does not complain about being ahead'
+
+# Without the flag, no lookup happens at all — the stub would report a newer
+# tag and must be ignored.
+f=$(online_fixture offline 0.8.0)
+_out=$(HOME="$f/home" DOCTOR_CHECKOUT="$f/ck" \
+       DOCTOR_LATEST_TAG_CMD="printf v9.9.9\\n" sh "$DOCTOR" 2>&1); _rc=$?
+if [ "$_rc" -eq 0 ]; then
+  pass=$((pass + 1)); printf '  ok   the default run performs no remote lookup\n'
+else
+  fail=$((fail + 1)); printf '  FAIL the default run performs no remote lookup\n       got: %s\n' "$_out"
+fi
+
+# A remote that cannot be reached is not drift and must not be reported as it.
+# An unreachable remote produces no output at all, so it gets its own run
+# rather than an empty stub argument — "prints nothing" and "prints an empty
+# string" are different things and only the first is the case under test.
+f=$(online_fixture unreachable 0.9.0)
+_out=$(HOME="$f/home" DOCTOR_CHECKOUT="$f/ck" \
+       DOCTOR_LATEST_TAG_CMD="true" sh "$DOCTOR" --online 2>&1); _rc=$?
+if [ "$_rc" -ne 0 ] && printf '%s' "$_out" | grep -q 'could not'; then
+  pass=$((pass + 1)); printf '  ok   --online reports an unreachable remote as its own fault\n'
+else
+  fail=$((fail + 1)); printf '  FAIL --online reports an unreachable remote as its own fault\n       rc=%s got: %s\n' "$_rc" "$_out"
+fi
+
 printf '\ndoctor: %d passed, %d failed\n' "$pass" "$fail"
 [ "$fail" -eq 0 ]