fix(crosscheck): load Azure fleet in Slack service

[ruby-dlee]

Summary

• load the coordinator owner-only Azure fleet environment during launchd preflight and listener startup
• reuse the Azure runner provenance-checked, bounded environment parser
• pass only allowlisted Azure configuration into Crosscheck children without copying values into the plist
• fail before service mutation when the fleet environment is missing or invalid

Verification

• bash tests/fm-crosscheck-slack.test.sh (28 units)
• FM_SLACK_TEST_GROUP=service-repair bash tests/fm-crosscheck-slack.test.sh
• actual coordinator parser load verified by variable names only, without printing values
• Python compilation, diff hygiene, and pinned ShellCheck 0.11.0