fix(#1408): mosaic-agent@ condition arms on either home shape

The reconciler writes seat projections into the brain home when one is active
(~/.mosaic/fleet/agents, mirroring start-agent-session.sh's brain-home
resolution) and into MOSAIC_HOME on a legacy single-tree host. The unit's
single config-home ConditionPathExists therefore skipped every seat on
brain-home estates — measured on two estates independently: 27 projections vs
0 at the config path, and 5 vs 0; the gate never fired on either.

Fix: two TRIGGERING conditions (`|` prefix — same-type conditions otherwise
AND, which can never be true across two paths). Either shape arms the unit;
start-agent-session.sh still resolves the authoritative copy itself.

test-fleet-units.sh now asserts both triggering lines and REFUSES a bare
(ANDing) ConditionPathExists spelling — that regression would disable
autostart fleet-wide while reading as "has a condition". Sabotage-controlled:
unit reverted, suite fails on the first new assertion; restored, green.

Closes #1408

packages/mosaic/framework/systemd/user/mosaic-agent@.service

@@ -11,7 +11,17 @@ PartOf=mosaic-tmux-holder.service
 # launcher would fail the unit. A skipped unit is the honest state for "enabled
 # but not yet configured"; systemd re-evaluates the condition on every start, so
 # the seat comes up on the next start once the reconciler has written env.
-ConditionPathExists=%h/.config/mosaic/fleet/agents/%i.env.generated
+#
+# #1408: the reconciler writes projections into the BRAIN home when one is
+# active (~/.mosaic/fleet/agents, mirroring start-agent-session.sh's brain-home
+# resolution), and into MOSAIC_HOME on a legacy single-tree host. A single
+# config-home condition therefore skipped every seat on brain-home estates —
+# measured on two estates: 27 projections vs 0, and 5 vs 0, gate never fired.
+# Two TRIGGERING conditions (the `|` prefix ORs same-type conditions, which
+# otherwise AND): either shape arms the unit; the launcher still resolves the
+# authoritative copy itself.
+ConditionPathExists=|%h/.config/mosaic/fleet/agents/%i.env.generated
+ConditionPathExists=|%h/.mosaic/fleet/agents/%i.env.generated
 
 [Service]
 Type=oneshot

packages/mosaic/framework/systemd/user/test-fleet-units.sh

@@ -32,6 +32,17 @@ if grep -qF -- '/bin/bash -lc' "$HOLDER"; then
   fail "holder must not start tmux through a login shell"
 fi
 grep -qF 'Requires=mosaic-tmux-holder.service' "$AGENT" || fail "agent does not require holder"
+# #1408: the projection condition must arm on EITHER home shape. Both lines must
+# carry the `|` triggering prefix — same-type conditions without it AND together,
+# which can never be true (one file cannot exist at two paths), so a bare-spelling
+# regression would disable autostart everywhere while reading as "has a condition".
+grep -qF 'ConditionPathExists=|%h/.config/mosaic/fleet/agents/%i.env.generated' "$AGENT" || \
+  fail "agent lacks triggering condition for the config home projection"
+grep -qF 'ConditionPathExists=|%h/.mosaic/fleet/agents/%i.env.generated' "$AGENT" || \
+  fail "agent lacks triggering condition for the brain home projection (#1408)"
+if grep -qE '^ConditionPathExists=[^|]' "$AGENT"; then
+  fail "agent has a non-triggering ConditionPathExists — same-type conditions AND, re-arming #1408"
+fi
 grep -qF 'start-agent-session.sh' "$AGENT" || fail "agent unit does not call start-agent-session.sh"
 if grep -qE '^Environment(File)?=' "$AGENT" "$INTERACTION"; then
   fail "agent units must not accept ambient or projection environment before strict parsing"