ci(#1297): wire the broker-preflight suite into test:framework-shell; track the env contract

Pipeline 2468/2556 failed the CI 'sanitization' step; the log (2556, step
60166) shows verify-sanitized.sh PASSED both times — the failure was the
test-enumeration guard bundled in that step: the new
tools/fleet/test-agent-session-broker-preflight.sh existed on disk but was
not enumerated on any CI surface. Wired as the last link of
test:framework-shell (guard: population 62, enumerated 47, excluded 16).

Freshened onto current next, which also grew a required generated-env key
(MOSAIC_GIT_IDENTITY, = agent name): the suite's hermetic env.generated
fixture now declares it, keeping the suite CI-fit against the current
start-agent-session.sh contract rather than the one it was written for.

packages/mosaic/framework/tools/fleet/test-agent-session-broker-preflight.sh

@@ -55,6 +55,7 @@ chmod 700 "$FAKE_HOME/.config/mosaic" "$FAKE_HOME/.config/mosaic/fleet/agents"
 chmod 750 "$FAKE_HOME/.config/mosaic/fleet"
 cat > "$FAKE_HOME/.config/mosaic/fleet/agents/preflight-test.env.generated" <<'ENVEOF'
 MOSAIC_AGENT_NAME=preflight-test
+MOSAIC_GIT_IDENTITY=preflight-test
 MOSAIC_AGENT_CLASS=worker
 MOSAIC_AGENT_RUNTIME=pi
 MOSAIC_AGENT_MODEL=