fix(make/act-swarm): let the zombie drill ask discovery for swarm rows

swarm-zombie filtered the act matrix on mode:swarm but left the workflow's
modes input at its default, so discovery drew rows from every mode and
assigned each one by its position in the candidate list. For svc-db-postgres
that put variant 0 on compose and variant 1 on swarm, while the matrix filter
asked for variant 0 on swarm - a pair that assignment never produces.

act then had no matrix entry for the deploy job, ran discover alone and exited
0. The drill never happened, which is what roundtrip.sh's completion-marker
guard reports:

    swarm-zombie exited 0 but the drill never completed for svc-db-postgres

Two filters were deciding the same thing from different ends: ACT_MATRIX picks
from the finished rows, ACT_INPUTS decides which rows exist at all. Pinning
modes=swarm makes every row of the whitelist a swarm row, so variant 0 is one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Makefile

@@ -717,7 +717,7 @@ swarm-zombie: install-act
 	 ACT_WORKFLOW=.github/workflows/call-test-deploy.yml \
 	 ACT_JOB=deploy \
 	 ACT_MATRIX='apps:$(app);variant:$(or $(variant),0);mode:swarm' \
-	 ACT_INPUTS="whitelist=$(app) distros=$(SWARM_DISTROS) index=0 sweep=0" \
+	 ACT_INPUTS="whitelist=$(app) distros=$(SWARM_DISTROS) index=0 sweep=0 modes=swarm" \
 	 bash scripts/tests/deploy/act/workflow.sh
 
 .PHONY: system-purge