feat: Enhance Mattermost integration by adding sync commands and context checks for recent messages

.opencode/commands/mattermost-sync.md

@@ -13,6 +13,8 @@ Run the command and use its output as fresh communication context:
 
 !`if [ -n "$FIDELITY_MATTERMOST_SYNC_CMD" ]; then bash -lc "$FIDELITY_MATTERMOST_SYNC_CMD"; elif [ -f scripts/mattermost/sync.sh ]; then bash scripts/mattermost/sync.sh; else echo "No Mattermost sync command is configured."; fi`
 
+Use this command implicitly when the user asks for the latest or last Mattermost message, especially messages from Jeff or the current manager.
+
 Then:
 
 - if the command fails, stop there and do not edit any workspace files