regression: `watch.sh` no longer folds sentinel `-`

[apstndb]

Summary

#477 taught callers to pass "${GROK_SESSION_ID:--}" (and the same
pattern for other hosts) so a launcher that drops a quoted-empty first
arg cannot shift project/type. watch.sh was supposed to fold the
literal - into the empty-session-id path.

On current main the callers and grok-build _delivery.sh comments
still describe that contract, but watch.sh does not fold. The bats
case that locked it is also gone. This is a regression of #477, not a
missing caller fix.

Do not change grok-build "${GROK_SESSION_ID:--}" sites.

Repro

Join as grok-build, then (no session id in that argv):

bash scripts/watch.sh - /path/to/project grok-build

Expected: run/watch.agmsg-*.pid or a grok composite id; no
run/watch.-*.pid.

Actual on main / v1.2.0: run/watch.-.pid.

Observed 2026-08-18: vanilla v1.2.0 via grok monitor tool (session
var set in the grok Shell, unset in monitor re-eval) produced argv
watch.sh - … and watch.-.pid. Same host, fold restored: no
watch.-*.

Suggested fix (one PR, one file)

SESSION_ID="${1:-}"
[ "$SESSION_ID" = "-" ] && SESSION_ID=""

Restore the bats case from #477 in the same PR if CI should lock it.

Related

• fix(watch): fail loudly on shifted args instead of running with zero subscriptions #477
• Independent of pid-walk (_agmsg_agent_binaries).

[apstndb]

Implementation: #857